I'm not normally that bothered about keeping my strings amazingly-super-bright but I have always wiped down after playing with Fast Fret. I kind of feel like it does something, but maybe it's just placebo. However recently a few things have happened all at once which has got me really thinking about the life extender type products. Firstly, I've joined a pop punk band and need the brightness. Secondly, strings are ridiculously expensive nowadays. Thirdly I've sort of hit a concentration of needing new strings. It's like this year when the 5 year and the 7 year cicadas will line up, several of my basses would all benefit from new strings at the moment, to an investment of around £120. It's be nice to keep them lasting as long as possible.
I have used Fast Fret, Dunlop XLR8 (that felt useless) and more recently the Dr Duck's Ax Wax, which I kind of feel like it does something. Anybody else swear by anything that I might want to check out?