Right if you want ideas on how to build muscle mass I suggest you go and look at
www.bodybuilding.com it's an excellent site with a wide range of training ideas, nutrition advice and columns from experienced bodybuilders, powerlifters, coaches and nutritionists.
Without a gym to work in your options are severely limited as lifting heavy is the way to gain size. If you can get 12 reps out then you're not lifting heavy enough for hypertrophy (muscle size increase) you really ought to be lookin at 3 sets of 6-8 reps and once you can get the 8 reps on your third set then next work out raise the weight. If you're training like this then overtraining can occur however if you're only working with body weight and cardio work then overtraining is not likely to be a worry.
Also if you're training heavy weights and not gaining size then you're not eating enough and certainly not enough of the right things. A good idea for body building is a ratio of 50% carbs 40% protein and 10% fats (ideally Omega 3 oils plus some other animal fat) and as a guide for how much you should be eating you should bear in mind that you should take 1.5 grams of protein for every kilogram of body mass you have.
Brian if you're only using body weight then you should try varying the hand position on push ups, use a very wide stance then a very close stance (thumbs touching) and dips so you can work your pecs and tris better than just push ups, also try doing push ups with one leg raised (I prefer it to the raised feet idea but that's cos my wrist gets really sore), also try som wide arm pull ups, instead of having your hands close together try them so your hands are about 8 inches outside your shoulders with palms facing away from you.
Do body weight squats and lunges as well and try to do calf raises on the stairs (if you only use one leg body weight isn't a bad weight).
There's loads of other good isometric excercises you could do to tone and increase endurance but they wont really help you pile on mass.
