Hold a care package/sentry gun/EAD grenade in your hand while standing by any wall that says to hit A to jump it.

Hit A, as he climbs it, pause for a second and hit Right on the D-pad to bring the grenade back up (otherwise he switches to gun).

If you timed it right, the grenade will be in hand. Throw the grenade, use the killstreak, you get the grenade back/repeat.
Note that if someone takes the package or if ammo is the last one taken, the killstreak rewards will be unavailable until you reach a higher killstreak or die, at which point the others will be available again.

Great for boosting CB (I only boost offline splitscreen, don't hate me).