Always use full cover, even if it makes you move in a little slower.
If a sniper isn’t getting hits, dump that fucker and hire a new one.
Blowing away cover with a demoman, so the rest of your team can lay in to the exposed enemies, is probably the single most useful ability in the game.
When you get the option to have your demoman carry two grenades, have them bring one flashbang and one grenade. Flashbangs have huge range, do not affect your own people, and make it basically impossible for enemies to land a hit.
When you get telepaths by building that telepath training building, go for mind control above all else. Andromedans and other big dumb a-holes are excellent for drawing fire and distracting, all while damaging one of your enemies.