**edit: Speech alert..**
just though id share a little about my beat-making process. As far as making beats goes, i am definitely really far from where i want to be. However, take from this waht you will, it may be useful.
i used to have that problem a lot, maybe a year ago when i first started. (im 17 now)
i would have a nice loop goin, and i would have no idea what else to add. i still get stumped a lot. that thing about leaving and coming back later is a good idea, but some6times that doesnt work for me. so heres what i like to do at that point...
if you are using a sample that u dont know the key of, then load up a simple piano and play around on the keyboard a bi untill you find the first note of the loop/phrase. then just go from there and play around, and find as many little "riffs" (i play guitar, dont know what else to call them) as you can.
mute the ones you make, and then go on making them. make a bunch; like ten or something all different.
during this time, if you hear anything on the piano that sounds good, that you think would imporve your beat, then work on adding that in. maybe you hear the first few notes of a solid bass line, or a lead line, or whatever.
if somehow you sont come up with anyhting at all by doing this, then i would suggest turning the beat off like pccjamie said, then come back. when you are listning to it again, try to envision (or enheearing??? lol ) somethinat that would imporve the mix.
then try to lay that down, out of your head, on the keyboard. i usually have had som,e good success with this, as i have some pretty good keyboard skills from my classical piano education.
another thing that is important or certainly helpful to do at the end is to go through and add maybe some crashes and other percussion sounds for dramatic effect.
good luck, keep ya head up.