Is there an average life span for a preamp?

When your pre or amp no longer has the sparkle, it's probably the caps. I just changed some 15 year old caps, and that prvented me from buying a new pre.

I have seen pre's from the late 70's and still sounding good...it depends on how you treat them.

How do I treat the capacitors well besides providing enough space for ventilation? Do I talk to them and say that they are good boys and girls? Any suggestions?

Filter capacitors will hold up better if they continue to get used on a regular basis. However they will still degrade. Any piece that is over 20 years old should be considered suspect. It may well still work, but that is not the same as saying that it is working as well as it should.

Atmasphere, My calculation based on Nichicon data shows that at 35degC (95degF) standard caps should have 80% of capacitance after 17.8 years. Each 10degC cuts life in half, so I would estimate not more than 10 years for caps in hotter amps unless 105degC or long life caps are used. When ESR of the cap increases to cause substantial heating from ripple current they get even higher ESR (since ESR increases with temperature) and thermal runaway causing to blow fuse or scored top. It happens when capacitor is very old and starts running hot. It should be easy to detect such caps especially since there should be other symptoms like less dynamic sound, poorly defined bass, 120Hz noise etc.

I can understand that many audiophiles have "gardener's syndrome" (constant need to trim and re-pot) but if it sounds good to you - don't touch it.