If you find yourself in a poor position when you're being closely attacked on foot by an enemy, you can use corners to your advantage. If there is a corner available, turn it and quickly toss a frag against the wall (the one pertaining to the corner you just rounded). Preferably, you should toss it as close to the bottom of the wall as is feasible. Of course, you should keep moving as you don't want to get caught in your own blast. The idea here is that the enemy pursuing you will be so greedy for the kill that he/she will walk right into the blast, giving you an opportunity to finish them off with whatever weapon you have. I can't tell you how many times I have turned the tables on the opposition using this technique.
You can also use frags to quickly finish off an enemy with a surprise attack. If your within close proximity to an enemy who is oblivious to your presence, try tossing a frag at his/her feet before shooting. The blast plus your fire should make quick work out of them.
One last tip. Don't underestimate the long range capabilities of your nades. Experiment on different maps and during game play to see just where and how you can utilize nades (frags in particular) to either get a kill or blast a weapon towards you from long range.