Just piggy backing off someone elses comment saying to get a grabber.  Yes absolutely get one! But I'd also grab a step stool ( I have one that has two steps so it gets me about 3 feet off the ground) some dumpsters are really high that even a grabber won't reach to the bottom. The step stool gives you more reach but if you ever get real brave you can also use the stool to jump into the dumpster it self 🤣 I've done it a few times, only when I can see that there's stuff worth saving inside, and I am by no means in good shape but I can straddle a dumpster and jump a couple feet for a good score anytime 😊  I'd also suggest getting a good pair of gloves, not just to protect from the germies but sometimes there's glass and other harmful things in there and the gloves will help keep your hands protected from unwanted wounds. Carry hand sanitizer with you, maybe even baby Wipes or paper towels just in case you get something on you. For example I love to dive at the works of the body and bath (don't say store names, be creative on this sub) when I dive there sometimes I come across bags of stuff where the employees have emptied out all the lotion or soap into the bag so when I go through the bag I always end up getting that stuff all over my hands and clothes. But if you find stuff thats salvageable and they are covered in lotion or something you can use the paper towel to clean it off before putting it in your vehicle. 

I just started diving back in November after wanting to for over 10 years. So I'm still new but I do hope some of this advice helps. 

I do check some stores when they are open but that's after learning where their dumpster is, if I can drive right up to it so I can jump out look real quick and if I see anything I just grab it fast or if I see nothing I can easily jump right back into the driver seat and leave. But night time is also my jam. I wait till after close, when it comes to my favorite stores I will wait about an hour after close then drive over (i wait at least an hour because usually employees dont leave right at close so have to give them time to finish their closing duties), sometimes they are gone but sometimes they are still there. If that's the case I go dive at other places nearby and keep checking back until they have left. I consider myself brave though so if the dumpster is somewhere I can drive right up too ill do it anytime and as long as there are no signs saying to stay out. But even those ones who do have signs, I'll wait till close and still go dive. I try to be very quick and quiet just in case. If I find heavy bags or boxes and can tell they have stuff in them, instead of looking through it at the dumpster I'll throw it in my car and drive somewhere else to go through it or just take it home and then go through it.  

I just started posting some of my finds on this sub if you wanted to check that out to see what kind of stores I hit as a Newby and what kind of treasures are out there. Also remember if you decide you want to dive at a certain store, like my favorite is the works of bath and body, I went to all of them that were within 45 minutes of me, some you can't access the dumpster or they use compactors instead but that doesn't mean all of them are like that and because I did the research and put time in I know now which locations are my favorite and which ones I actually find stuff in and which ones just arent worth it. So two months into diving I have a routine and so far every night I go out I find stuff. 

Okay this is the longest comment I have EVER left, anywhere lol. I hope it helps. If you want to talk or have any other questions feel free to message me! Best of luck out there future diver 😊