A couple of drills i find useful for developing the slip are;
1 kneel opposite a partner just inside his/her range(they are also kneeling BTW) and work for a round with your guard up allowing them to throw straight punches at you. your job is to slip them. The kneeling takes the footwork out of the equasion. experiment with rotating as you slip to 'load'.
2. Stand about a metre and a half away from a wall - you must be facing the wall - in your boxing guard. then get your partner to throw a tennis ball against the wall -from behind you - in such away that after it bounces off the wall it comes straight for your face. then slip it !!!! works the reflexes well.
hope they help.