Narrate your gameplay. Talk about where you are, where you're going, what you're doing right now, what you're planning on doing. Mention if you're fighting someone or leave, if you're getting ammo packs, etc. Pretend you have someone sitting behind you whom you're explaining everything to. Someone listening in should be able to know exactly where you are and what you're doing without even watching -- record yourself and listen, or have a mentor listen. You can always tone down clutter later.
Also, put your PTT on something where it doesn't affect movement keys, jumping, crouching, swapping weapons, or aiming/firing. Shift works really well if you're using WASD+Ctrl+Space for movement and weapon selectors on the mouse. You don't want to be avoiding talking just because you can't without losing a fight or fucking up a jump.