Black Diamond strings. These are black coated strings. Last for months (because even the unwound strings are coated, unlike Elixir, which I like) and never need cleaned. There's no gunk that builds up on them, at least not in any noticable way.
If you love your strings though, there are 2 products that I have used and work pretty good.
1. GHS
Fast Fret
2. DR Extra Stringlife - Liquid Polymer Coating
Both of these will allow you to get a little extra life out of your strings, although Elixirs last a LONG time (apart from the unwound treble strings) and Black Diamond's last even longer (as I said, even the treble strings.)