The post above by RTCdude is very well written and should be consulted by anyone wishing to fix a remote.
My experience cleaning remotes is mostly confined to yearly repair of a TV remote.
After about 10 years, it needed regular reconditioning.
If it hadn't been so unusual, I would have gotten a spare or used a universal.
I'd clean the PC board contacts and the worn carbon buttons on the silicone sheet with alcohol and a Qtip, let them dry , and reassemble.
Problem is, once you start cleaning them, they need it regularly.
A lot like cleaning mouse balls - by the time they need it, they're pretty well worn out anyway.
(Yes, I know there's a joke in there...
)
Oh, and I picked up some spare PVR 501/508/510 remotes last week for $5 each.
edit:
It was at an electronics swap meet, held every other month in my area.
The units were used, but I picked two that looked new.
DP34's were $15?... DP LNBs around $10?.... even found the 121/119 LNB for $20.
By comparison, the used remotes I'd bought on Ebay for $12..15, looked like crap.
One thing you don't want to do, is clean the force-stick (mouse replacement) in the middle of an IBM (or similar) keyboard.
I did once (with alcohol), and the rubber eraser turned to sticky goo.
I had to find a replacement.