Transistor/FET Substitution Guidelines?

[DaDA SaLAMaNDeR pOLyP]

Are there any rules of thumb one can follow when substituting transistors 
or FETs in a circuit?  I know that you can change resistor biasing, but 
what about other stuff do you look for?  I find old tranistors in 
databooks but don't know which values are important.  I suppose the 
application makes a difference too, such as whether the transistor is 
being used as a switch, amp, etc...

Any tips?

Thanks,

Romeo
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ms20 at wolfe.net