Here's a spanky new product (ok, maybe not to you, but it's the first time we've ever seen anything like this) from Farnell - http://uk.farnell.com/mega/600-020/tin-plating-powder-90g-for-1-litre/dp/1550282 - it's tinning powder for homebrew PCBs. You might be able to get the same stuff from Rapid, but we're Farnell diehard fans here, though you may find the same stuff cheaper on eBay.
Wherever you get it from, make sure you buy the stuff that works at room temperature. Dissolve the crystals in warm water (the instructions recommend 50degC but I just made a brew, drank it, gave it a few more minutes then used the left over hot water from the kettle.)
Why bother tinning your homebrew PCBs?
Well, firstly, it makes them easier to solder to. We use loads and loads of flux for the same reason, but as you can see from this board (below) the exposed copper tarnishes quite quickly. Tinning helps reduce tarnishing on your finished boards (and gives them a nice shiny coat which looks a lot neater!)
About twenty quid gets you 90g of powder which, according to the instructions, should make a litre of tinning solution. In powder form, the compound has an unlimited shelf-life. Made up, it's expected to last about six months (but they say that about Ferric Chloride and we're still using the same batch nearly eighteen months after making it up!)
Just make up the solution and drop the board in.
We left ours in for less than ten minutes, face side up. Rather than use the whole lot in one go, we made up about 200ml and just added a bit of powder at a time, stirring until it had dissolved. The result was pretty impressive:
Nice shiny silver traces make for easier soldering!