Simple ways to overcome a "plateau"?

I have been lifting free weights for 3 months and so far all i have been doing is curls and bench press....which i do simultaneously every other day. I also threw squatting into my routine. My equipment is limited...only a rusty old bench and a dumbbell with the usual ortment of weights for it. While i am clearly getting stronger and able to lift more and do more reps at heavier weight i am concerned about being stuck on a plateau. My main interest is muscle m, i dont want to diet although i do try to eat things like eggs, tuna fish, beans potatoes ect.. for protein. Other than the position of my hands on the bar, what are some tips to help shock my muscles into getting stronger? And also some other tips would be good...i am a serious lifter but with limited means, financially you could say.