suggestions for a car that has sat for a bit

Hello.

I have a 1998 automatic 2.3 Cl, 250,000+ miles. it's started having problems with shifting and all that, so we're planning on replacing the transmission. The car started having issues last year in Oct, and I decided it was unusable as a daily driver in late Nov. poor girl hasn't seen a lot of action since then, so here's my question...

Any tips or suggestions of things to do to after a car has been sitting for a while so that she gets back into fighting shape? Should I change fluids, or use anything in the engine to clean out affects of spending too much time sitting around? Or really just any tips on how to maintain a high mileage Cl?

Thanks!