Lay traps and don't care about winning. Use C4 for controlled explosions (and as was noted above grab kits off of dead players to maximize effectiveness) trap flags, heavily patrolled routes, lay C4 traps on ladders, claymore traps on the tops of ladders, stick C4 to walls and behind corners/obstacles where you know enemies are going to be, when capping flags that have a few entry routes (ie. inside doorways or at the tops of ladders/stairs) place C4 at those areas and detonate when enemies appear, place claymores by vehicles or in the middle of roads to get passengers in Vodniks/HMMVs, use suicide tactics, put C4 on vehicles you are driving and leave them for enemies to pick up (works well with Tanks and APCs), put a coupla C4s on enemy helipads or hangars, be devious!