Give a new life to my 2007 MacBook Pro (help it live longer)

Well, if it's a MacBookPro3,1 model, then you can upgrade the RAM to it's maximum of 6 Gb. Ram is extremely cheap these days, would be less than a 100$ I imagine.

As for hard drives, you could install a 7200 RPM drive (like Western Digital Black series), again cheap, ~100$ for 750 Gb. If you have a bigger budget and no need for such space I strongly suggest installing an SSD drive, at least a 128Gb. This is by far the best upgrade option you can do on your computer. You won't believe the difference in speed! Check out OWC's Mercury extreme 3G's SSDs from Macsales. They are by far the best out there.

And I would install at least OS Snow Leopard, it's pretty solid and fast. Can't say for Lion yet, haven't tested it.

As for software there isn't much else you can do. Reinstalling the entire OS from scratch will bring you back to it's original speed but machines tend to slow down over time, this is normal. In my honest opinion, the main problem these days is the hard drives that can't keep up with the rest of the computer. This is why an SSD is a good solution to remedy this problem. And if you buy a bigger version or better yet a SATA3 version of the SSD you could transfer it to any new laptop you might buy in the next 2-3 years, it will be even faster.

I would upgrade to Snow Leopard. It's definitely faster. Also, check out this article. Another thing that I have found helps is to restart your Mac into Safe Boot Mode (Startup while holding the shift key; when the gear starts spinning you can let go. It'll take about 7-10 minutes to boot up). Once it's booted up, restart again (without holding any keys down). Here's an Apple doc that explains what Safe Boot Mode does.