I have been weight training constantly now for about 3 months and I have definately reached a plateau where I cant seem to lift any more weight without hurting myself (shoulder joints, back).  I workout in a home gym with a bench, bars, dumbbells.  I have improvised mostly with what I have to try to get all of my upper body covered.  I think joining a gym would help definately, but I dont have the time for commuting there at the moment.  Any suggestions as to what will help me break this barrier?

First thing I'd do is take a week off. Maybe a little bit of lite cardio but no weights. Change your routine. If youer using barbells for presses use dumbbells ect. Change your days.Do your workout in reverse.

Make sure you're not over training. No split routines(upper body one day lower body the next) work the whole body each workout. Start with the largest muscles first decending to the smallest. (biceps should be your last to work.) No more than 45 minutes at a time three days a week. Try slow reps. Nothing faster than 2-seconds up and 2-seconds down. To shake things up try 5-seconds up and 10-seconds down to work the negitive side of the lift. No more than about twelve exercises for the whole body. One set per muscle or muscle group. Always go to momentary muscle failure and try for one more. Use perfect form and full range of motion. When you can do twelves reps with a given weight it time to add 5% to that exercise at the next workout.Take an extra day off in between workouts. Drink lots of water. (0ne gallon minimum). You must always be progressing, adding either reps or weight.

Creatine. Helped me work through my plateau, you should also check your diet and make sure your getting enough protein. Keep pumpin.

Assuming that you are in good health and have no injury(s),illnesses,or disease(did you get a physical recently),and also assuming that your diet and nutrition is adequate...your comment that you have been exercising constantly leads me to suspect a number of things you may be doing wrong...the first concern is possible overtraining...are you allowing your body/muscles enough rest and/or recovery time between workouts?...I have made frequent and numberous replys on this site in this forum particularly about training principles and exercise science...including stuff on overtraining...also your body will have a natural cycle/rhythm of rise and fall with regard to strength,endurance and the like...it is normal for the body to plateau when it has reached its natural maximum potential...only later to again show gains...the body also will habituate to the same repeated demands over time and level off with out gains until changes in the demand/routine are introduced...you dont want to exercise all the time...and not always in the same way...make sure you are well(see the Doc if in doubt),maximize your nutrition,cleanup your diet if indicated,and vary and mix up your training...add cross training with different activities/sports/exercises to keep the body on its toes and challange the body to continue to adjust to there changing demands...your mood and interest will most likely improve with this added variety as well...it is incorrect to think that you can continually push your body and receive constant gains...development is not linear...it will go up and down...we also have our limits...you ..may not have reached your maximum potential but the rate of gains will slow down as you approach it...the further up the ladder you go the slower and harder the accent...patience and perseverance are required...educate your self and apply your knowledge to your workouts...if you cant go to the gym(if you do talk with a trainer to help you with a correct program)then read some more books on the subject...I recommend books on fitness and health...not just stuff on weight lifting and or body building...there is alot of crap out there and alot of stuff people say and do and preach is wrong,old school and not only of limited value(especially if you are training to improve motorcycling skills)and potentially dangerous...body building and sculpting asside(also powerlifting)...the kind of things you need to do/should be doing to be a better rider/racer are different...Sincerely Doug

You're overtraining man!!!! Read what Hotler said. . that's good, sound advice. Also, note that you have not been training long enough (3 months) to "plateau". It sounds like you are simply over-training. Make sure you don't work the same muscle group every day, and eat well and get plenty of protein and especially water.
In time (6 months or more), you will see the benefits you desire. It takes a commitment. This is not a quick and easy thing to do. Stick to it, even if you get lazy, just do it!
And never go back to lighter weights, once you achieve a goal. Continue to add weight as you are able to. Note that gains come slower as time passes, don't get discouraged, this is normal. Just keep on pumpin'
L.L.