best way to run stable or testing program on ustable system

[Grzesiek Sedek]

Hi,
I run my system as unstable, but in certain situations (i.e. recent
Evolution Segfault oo Mozilla problem) I'd like downgrade certain
programs back to testing or stable - until the problem is resolved. I know
that it is possible so could anyone point me to a document about it or
show me their way to do it? 
Thanks