Would help if you told us what you have now, brand and especially size.
Larger sizes are more troublesome.
I’d say a 16GB Fat32 formatted stick would be your best bet, filled with high-quality MP3s that would give you about 1.600 tracks. Plenty for even the most grueling 8-hour set I think.
Our friends over at DJ Tech Tools have done an article earlier this year on just this topic.
http://www.djtechtools.com/2015/01/11/digital-crates-what-are-the-best-flash-drives-for-djs/
Hope that helps.