A few suggestions: B-5 helps thicken the new hair that comes in. The common dosage seems to be 1000mgs a day in split dosage (500mg am, 500mg pm). Also I would suggest trying a gentler relaxer and leaving it on for a shorter period of time. This way there's some wave left to the hair which allows it to maintain some of it's natural thickness.
For an immediate fix, I would suggest using a thickening conditioner and a leave in. nexus headress is a great thickening leave-in. Also airdrying your hair will leave it looking thicker. hth