It will depend a little on the instrument. For example, rhythm guitar with lots of strumming will be slightly different from picking out a melody on, say, a clarinet. Yes, the rhythmic pattern will be the same, but its execution is different, as is reading chord symbols as opposed to dots.
Tapping a foot is always good, as is internalizing the basic beat with a part of your body. My violin teacher used to sniff in place of rests, to keep them in time and place. (Christine Perfect's (McVee) dad, namedropping!) It worked well, but didn't sound too good.
Metronome will get a mention - sometimes it can be set for twice or thrice the speed to keep more accurate time.And of course, counting out loud. 1e&a 2e&a et al.
Put the radio etc on and tap along using different patterns, there are literally thousands to choose from, and visualise them as you tap. 'This is 4 quavers, a crotchet, 4 quavers and a crotchet' sort of approach.The good thing about this is you don't need an instrument - it could even happen as you drive along. Explaining to the law may be tricky!