try using some very fine sandpaper( say 1,000 grit or so ). this will usually do the job. stubborn marks or dirt can be removed with coarser sandpaper but you'll have to remember to finish with fine sandpaper or you won't slide enough.

the approaches at my house are so bad i have to clean them after every time i bowl. i use a flat rubber heel on my dexter sst 6's and when i'm done they're black from all the grime. first i use a emery board and this gets most of the gunk off. then i use a green scotch-brite pad. then 1,000 grit sandpaper, then 2,000 grit sanpaper. i finish it off with a 4,000 grit abralon pad. for a very smooth surface. this may seem excessive to some but if you saw the amount of gunk on my heel and the problem i have with sliding you'd understand.