hi. im making my characters with zbrush and im going to add more details on face but i need more polygons there and deviding putts more polygo in whole bodyand hard to continue to work and unusable for game. what is the best to do? thank you for helping

as ashaman said:

Sculpt high-poly model, retopo and bake to a lower poly model, use the low poly in game.

I am no ZBrush user, but I use 3D Coat, which has a very intuitive Retopo room for exactly that task. Blender has similar functionality, I guess ZBrush will give you a similar option.

This retopo room allows you to create a low-poly model on top of the high-poly one, working completly manually, or letting the tool generate a mesh from rough guides.

Of course you can also create the low-poly model differently, with poly-reducer routines, or creating a new mesh from scratch.

The baking process to get the High-poly details baked to the lowpoly model is an art in itself, not because it is hard to do, but because of the many baking errors provoked by ill-placed lowpoly geometry and UV layout.

There must be thousands of vids out there explaining the retopo process in your 3D Package of choice on youtube and other sites, so google around...

About your problems with working after increasing the polycount: As said I am no ZBrush expert, but besides what ashaman said there is AFAIK als the option of hiding parts of your model while sculpting. So you can hide the body while working on the head, which will reduce the amount of visible polygons a lot!