Generally, manual downshifts are no problem. If you're going to be downshifting hard (high rpms after downshift), I'd recommend rev-matching it by giving it a little gas during the downshift to reduce the shock load on the transmission.
the reason i have a automatic trans is so i dont have to do anything. to get to trans to live a long time, i put 2-3 qts of rear end grease in the trans. and the shift is a little tighter. but the best way to get it to live a long time is a shift kit.