Check the key isn't worn before anything else; they often wear quicker than the barrel and tumblers inside the lock. Have you got another newer or less well used key to hand?

To clean out the mechanism you really need to take it apart. Contact cleaner or brake cleaner will rinse out any crud, but it's difficult to flush out unless you can invert the barrel! Make sure it's fully dry after, then use dry lubrication such as graphite powder. (Graphite powder can be bought from lock-smiths and decent old-school hardware shops.)

Try and avoid the temptation to dowse the whole thing in WD40. It will help in the short term, but leaves a sticky residue over time as it dries, attracting dust and dirt like a magnet, eventually making the lock stiff and causing additional wear.

__________________