Leaky Valve Covers

[75sierraclassic]

I did a quick search and didn't find any info on this, sorry if its already been covered. What do you guys do to get your old style valve covers (side bolts) to not leak. I've tried just massive amounts of RTV (didn't work), I bought new more rigid aluminum valve covers and a high dollar Fel-Pro rubber gasket (also didn't work). It seems like it will seal up for while then start leaking again. I'm tired of having an engine compartment that looks like crap, there's oil all over the place from years of slow leaks. I know there is some way to do it, I just haven't found it yet.