I have an older MBP, it was bought new in 2008 so it could just be that I don't have the RAM to keep up with Lion, however, it takes my MBP around 2hours to boot!!! and then it freezes a lot! mostly when the system wants a password. Is there anything I can do without adding RAM or going back to snow leopard?
I am not sure only clean install can solve this issue. As you are using your MBP from last 3 years so chances of cluster of unwanted files are very bright. That why before going for clean install just try Speedup OS X Lion, a Mac cleaner software! Then go for clean install. In addition I would also suggest to check compatibility of the drive (volume) on which you are going to install Lion.