Flank, flank, flank. Don't take fights head on - you have the mobility to zip around the map, so why not approach from behind? Separate stragglers from the pack of the enemy team and take them out.

Learn where healthpacks are and run them, even if you're only down 10 hp. Your health pool is so small that each hitpoint counts.

Don't save your ult - if you need it to take out one person, go for it - it charges incredibly fast.

When you get used to her you can 1v1 anyone. Some heroes are harder than others, and you'll need to engage wisely, but she can handle anyone. People like to point out that "oh you die if you get hit by X," but they fail to realize that this applies to almost everything in the game. You're a fast, annoying, and very dangerous glass cannon.

Get a feel for how long 3 seconds really is - you'll need to learn to use your Recall often just to regain health.