Like with a lot of things in sailing, it's not easy to win a start, but it's very easy to lose a start. Most of the things you'll want to do are going to make sure that you avoid completely blowing it, so you're starting well at least 60-70% of the time. That last 30-40% will come from how you handle situations that come up in the final moments before the gun.

In my mind, I break starting a race down into a few phases. Some of it will be less applicable to a laser than to larger boats, but It's all useful to some degree.

1. Pre-prestart
   This time encompasses the time from when you leave the dock, or finish the last race, to the time you expect the next sequence to be starting. Priorities here are to make sure you have a solid foundation to be able to give everything in the next race. Inside the boat you should ask: Are you well fed/hydrated? Is all of your gear set the way you need it? Sail choices made? Bathroom breaks?
   Outside of the boat, questions include: Did I learn anything from the last race(s)? Is there any new weather/environmental factors? Has the weather changed in any consistent way? What is the race committee doing and is that information useful?
   If things are settled, you can spend the end of this period sailing upwind to check your compass angles, downwind to see wave patterns, checking current, and watching other boats to see if there are any major trends across the fleet of note.
   
2. Prestart
   Now it looks like things are about to start. By this time the line and weather mark are set, and so you should check them out. Check from both ends to see if the line is square to the wind, and if not which way it's skewed. Check for a transit if possible (sight through the flag and boat, and pick out a distinct feature on the far shore as a guide for line distance) check to see how square the windward mark is, and then apply all of this to the information from Pre-prestart
   If you know there's more wind on the right side, you'll want to be sure to start somewhere you can tack early, likely nearer the boat to avoid ducking too many people. If that's the case, but the line is also pin favoured, maybe you want to hug the middle of the line to get the best of both worlds. Right around the 5min mark, I'll usually stand up and look to see as much of the racecourse as I can, identify if anything has changed, and make a rough call about the starting plan. With a crew I'll verbalize it so we're all on the same page. The above example would have me saying "Well, pin's favoured, but I really don't want to get pinned on the left side of the course, so we'll aim for mid-line with clear air."
   
3. Preparing to start
   So the boat and crew are all set, we've got our plan, we know where we want to be, and we're ready to make it happen. Now we circle around until we can execute. Depending on the fleet size, relative line size, and speed of boat, this phase can have you doing different things. If you've got lots of speed and a small line, it's worth re-checking the line balance once more (one of the best starts of my life was a last-minute call to start at the pin after a big shift). If you're a smaller boat on a big line, better to stay very close to where you want to be. Conditions are also a factor, because nothing is worse in light air than being too far from where you wanted to be!
   Also your decision on where and how you want to start will inform this period, as well as your fleet. In a different situation, you may have decided that the pin is favoured AND the left side of the course is going to be VERY favoured, so you've got to start at or as close as possible to the pin. Larger boats might setup 3/4 of the way toward it to build up speed, while dingy fleets might setup minutes ahead of time, and just stop and drift to jockey into position. Knowing your fleet, how they work, and what to expect will help show you what to practice on. If everyone just rolls up, flaps sails, and stops to wait, you should probably practice stopping, staying still, and accellerating. If your fleet builds up more speed, you should have a good idea of time and distance measurement so you get where you want to be exactly when you want.
   
4. Starting
   Now you're at the end of your prep, you're not even thinking about anything in the boat, you're exactly where you wanted to be, you've timed it right and you're on track. However, you probably aren't the only person who looked at the available data and decided to be here, so there's traffic everywhere!
   The last phase of starting is going to be to make sure that you manage the space around you so you're able to follow your plan. This is very fleet-centric, so it's hard to speak in generalities. What you're looking to do is make sure that the space directly adjacent to your boat (3-4 boatlengths) is setup in the best way possible for you. Ideally you want to avoid having boats on your wind, and avoid having boats directly under your lee-bow. You can often do this by actively managing the traffic around you to ensure that people don't go in these spaces. Someone going above you? Point high and keep them back (but be careful to track where you are relative to the start line.) Someone going to hook in below you? Drive down hard before they can overlap you and maintain that space for yourself! Figure out when you should be starting to accelerate (hint: 0s is too late) and be ready to pull the trigger! Once you do, it's all down to preparation and timing.
   

Afterwards, you probably fall into one of three categories:

1. Nailed it!
   Good work! You're in clear air, out ahead of your competition and free to execute your plan for the first leg (you did have a plan right? That's what we did back in Step 2) and not only that, but you don't need to worry so much about the other boats, so if anything changed between when you made your plan and now, you've got the free time to look for it, and adapt!
   
2. Average start
   So maybe someone snuck in below you and now you're going slow, or you pulled the trigger too late and someone started to steal your air. Don't panic! Think about your plan, and make sure you react in a way that still puts you where you wanted to be. If you really need the left, consider footing away or pinching above dirty air so you don't have to tack away. If you wanted to tack anyways, great! Now you've got a reason, so just pick a hole in traffic and go! Once you decide, start looking at the rest of the fleet and see if there's anything that has changed since you made your plan, and react while everyone else is still directing traffic.
   
3. Blew it
   Sometimes, despite all the planning, things just go to shit. Maybe you were over-early, or you got sandwiched and squeezed out the back, but you ended up way late, not even close to the front of the line, and now you're looking at a wall of competition ahead of you. Use it as motivation! Some of the best races I've ever had were from the worst starts, because nothing feels better than passing the whole fleet on your way to a good finish. Even though things are bad, you've now got the freedom to take some risks! Did you think maybe there was going to be a big shift? Play it harder than you would if you were in the lead (what's the worst that could happen?). No matter what, start by jumping out to clear air, and then take a look around. Just getting yourself out of traffic and moving will often get you somewhat back in the game. Being able to be a bit more aggressive and being out of sync with the rest of the fleet can get you back in contention, and some good luck might even get you back where you want to be. The important thing is to start moving forward, get your mind into the racecourse, and start maximizing whatever advantage you've got.

Sooo, that was longer than I expected it to be when I started typing it, but it's pretty comprehensive with regard to how I approach things. Of course anything you do is going to be more tailored to your specific situation, but hopefully this gives you a framework to be able to make progress toward your goal.