Do you rotate between heavy weight/low reps and light weight/high reps? If so, how long do you wait between changing? I've been lifting moderate weight (basically what I can do for three sets of ten reps) for three months now,and am getting the feeling that I need to change it up a little. Just wondering how often you should change between heavy/moderate/light weight.

I don't change my workout that way.  What I am trying to say, is, I don't change from doing the same movement  with low reps and high weights to high reps and low weights.  I change the workout itself.  I do different movements.  I like to change at least every 6 weeks.  I found that this is the amount of time that my body takes to get used to the exercises and then get lazy.  There are certain exercises that I do not change.  These exercises work large muscle groups.   Things like, squats, and deadlifts, are kept  all the time.  
If you are trying to work towards having more endurance, you should design a workout for that.  

Heavy weights, low reps builds mass(with proper diet).
Lighter weights, higher reps tones and defines muscle shape.

The 2 basic workouts are individual muscle isolation exercises, and muscle group(tie-in) exercises.

Full range of motion in every exercise is critical to proper development.

If your hitting a wall try doing different excercises that hit the same muscle group. If you do alot of barbell work (flat bench, incline press, shoulder press etc.) switch to dumbell work(flat bench w/ dumbells, flies, etc).
What are your goals?
I know alot of guys at the gym that want to be ripped (and they are) and are not as concerned with strength and performance. Not to say they're not getting stronger it's just not their main focus. These are the guys working with lower weight with higher reps (8-12).

A power lifter doesn't care about the looks end of it as much as getting stronger(and bulkier as a result). They (we) hit the heavy weights for 4-6 reps max for most excercises.

The MAX-OT program will help you make huge gains if you want to get stronger and bigger. It's a long document but worth the read. (You will need to download Adobe Acrobat reader to read this):

MAX OT PDF DOC.

I am certainly not the guru of weight traing but this MAX-OT workout helped me personally. There's alot of supplements that he recommends that may be questionable but take it with a grain of salt.


Good Luck

I mainly lift to just to stay in shape and I always seem to feel better throughout the day when I lift. Getting bigger and more cut is a nice bonus though.

I have been doing some reading on workouts on the net and have decided to change it up every 6 weeks, I think that would work well for me. And im going to change between heavy/low rep (4-6) and light/high rep (12-14)

I just changed over to heavy weight today, so after this 6 week cycle, i'll switch to 6 weeks of light weight and see how that works and make some changes if I need to.