Subdivide and conquer.
This is what I did back in the day...after doing this, I automatically am always thinking in terms of 1/16th notes when I play, not really consciencly (sp?...always been bad at that.), but I always have my internal metronome
Start by breaking the bar up into as many subdivisions as you can, ie 1/16th notes or. So a 4/4 measure would be
1 e & a 2 e & a 3 e & a 4 e & a
Figure out how the beat falls into place, ie if it's straight 1/8th notes the notes would fall on 1, &, 2, &, 3, &, 4, &, etc. Say all 1/16th notes out loud (using a metronome) and clap or emphasize where the notes fall
1 e
& a
2 e
& a
3 e
& a
4 e
& a
So I am actually counting all the subdivisions out loud but only clapping on the bold things.
Then with a more complicated rhyth you get this:
1 e &
a 2 e &
a 3
e & a
4 e &
a
make sense?
This kinda covers the idea od subdividing.
and always start slow and work up to tempo.