Even though there are more formal counting techniques out there, I've always found it easiest to come up with a word (or words) with the same number of syllables as there are sections I want to divide the beat into. Speaking is pretty easy, and it doesn't take a lot of/as much thought for you to pace your words. You've been doing it your whole life. For example, counting triplets I say/play:
daf-oh-dil; daf-oh-dil; daf-oh-dil; daf-oh-dil; (four beats, divided by ';'s)
Now counting sextuplets would be:
daf-oh-dil_el-eph-ant; daf-oh-dil_el-eph-ant; daf-oh-dil_el-eph-ant; daf-oh-dil_el-eph-ant; (four beats, divided by ';'s)
Since the dil and the el sound somewhat alike, it becomes easy enough to run them together and you can get pentuplets pretty much for free by running the words together like so:
daf-oh-del-eph-ant; daf-oh-del-eph-ant; daf-oh-del-eph-ant; daf-oh-del-eph-ant; (four pentuplets)
Also, it helps quite a bit to practice with a metronome, and as soon as you can play sextuplets to the beat, slow it down some, then play a measure of 16th's then a measure of sextuplets, then a meausure of..... mix it up and learn the difference. Go back to straight one, then straight the other, then mix it up again. Your inner Ricky Bobby will want to go fast ASAP, but remember being able to play slow AND smooth is the fastest way to get fast.