Also, from Seitetsu on GF:
1) Every time you die, the opposing team gets money. All the Clunks that just explode and tank with no intention of staying alive.... stop it....
2) Many characters like to set up a combo. For example:
Froggy G - Dash into WW
Leon - Backstab / tongue traps
  Clunk - Chomp into explosion
  Yuri - Slowdown field
Lonestar's f-ing bull destroys all these setups.... think before you use it
3) You don't need to go back to base to get health every time. You can:
Get health that are scattered all around the map, kill neutral creeps, or if you just took a few hits, find a voltar. Every second you are out of the fight is less money you are earning.
4) Plan out your build. Do you really need those extra points where you are putting them? Is there something that you use a lot that should be upgraded before that? Builds can completely change how a character is played. Think about your options, and if you have the info, think about who else your teammates are using.
5) Don't step on the f-ing button when your teammate is over the wormdoor. It's not cute. See tip 1 and 3.  
6) If you are squishy, dont be a hero. Yuri and Voltar are team players. They aren't meant to go at it alone. If you absolutely must play Call of Duty MOAB, pick Leon at least...
7) If you have a burst damage setup, or are Leon, watch the map for distracted enemies. Is that lonestar really taking on a turret alone? Wait for the opponent to use their abilities, and immediately jump him while it is on cooldown. If you can, time the silence to give yourself a few extra seconds. Also, anticipate your opponents moves.
If you are Froggy G, your whirlwind can make your invulnerable. Use it to survive a self destruct or dynamite. Timing can make the difference in a flawless victory, or a fatality.
Use the environment to your advantage. Watch your map for people fighting around the worm door. If an enemy is coming and planning on grabing that gold solara bub, watch your map and drop from above right on the button right as he is in there. Free kill. Also, if someone is fighting the Slug boss, pop in and stun him and let the acid breath kill him. If you are on the small map, always activate your flying drones. They are more powerful than you think. Push people into turrets or use your stun right when they get in range of it so they take extra damage. Even if it doesnt kill, it takes them out of the fight long enough to get ahead.
Feel free to add to this.