If you find yourself having to slide back on the seat a bunch, it could be a few things.
- The nose of your saddle might need to be tilted up.
A level or slightly down nose will cause you to slide forward, which rests your weight on the nose of the saddle - which is probably exactly what you don't want (but often why people lower the nose). If the nose is raised, you stop sliding forward, and your weight rests on the wider, rear part of the saddle (which will usually be close to level if you look at that part and ignore the nose).
- Your handlebars might be too far forward.
You might be pulling yourself further forward because you're having to reach for the bars, then realizing your rear is too far forward and so you push yourself back again.
- Your saddle might be too far back.
It might be. But a too-far back saddle will present itself in other ways - knee pain, for example. Fore/aft and up/down saddle position should be determined solely based on how your legs interact with the pedals - get that right, and THEN adjust the rest of the bike to deal with other problems.