I have a fixed gear bike that I love riding, however I have a tendency to slide forward a bit in the seat. I've adjusted the seat to be all the way forward, but I still slide just forward enough that my sit bones aren't quite on the seat properly. I have a 90mm stem currently and was wondering if I just got a 60mm stem if that would help me to stay on the seat properly? or is it possibly just a bad seat?

Thanks!

Up angled one , can also help.. higher bars.. and closer to you..

by bringing the saddle forward you have taken body weight off the saddle

onto your hands

tried bringing the saddle nose up past level yet?

Is the seat to high? Try lowering the seat a small amount which should have you moving back.

I always setup my bikes with the saddle nose angled very slightly upwards to prevent forward sliding - something around 2-3 degrees at most. This is after dialing-in the correct fore/aft adjustment. You don't want to go too high with the angle to prevent unwanted pressure points. Grab a carpenters level, make sure the bike is on a level surface. If the seat post has grooves in the mounting plate, I usually only go one click when angling the nose up from level.

Wrong way. To fix sliding forward, move the saddle back. I never have the nose of my saddle up. Always level. Some folks have to move the saddle back quite a ways, even on a set-back post. It depends on personal body geometry. Ignore KOPS. It obviously matters not at all or else recumbent cyclists would all be injured.

I am also one of those that has the saddle very slightly angled up.
I don't think you should adjust fit by moving the saddle around. Set the saddle up in relation to the bottom bracket and keep it there. Then adjust your stem-height and reach. You may be sliding forward due to your seat angle, but also if your bars are too low for you. Depending on your core strength, you may not be able to hold your position well if the bars are too low. You can decrease the saddle to bar distance by raising the stem, not by lowering the saddle.

even a skosh of a degree helps sliding off the front , not a favorite ..

I am in agreement with Carbonfiberboy. I have been experimenting with the fit of a Focus Mares CX bike for a couple years now. As I have improved my physical condition, I keep adjusting my position lower and further back, which seems to allow me to be more aero but not feel like I have too much weight forward.