1997 Mazda 626 Check Engine keeps coming on

I have a 97 Mazda with 100,000 miles on it. It is a real good drive but for some reason the check engine light keeps coming on. I was told by a mechanic that the codes for the light suggested problem with the upstream O2 sensor. So I got both the upstream and the downstream sensors changed and it cost me like 190 bucks for the labor alone apart from the 60 bucks per sensor. The codes were reset and my check engine light went off and my car was running fine for three days after the fix. But this morning on my way to work all of a sudden my check engine light came on again. I am looking for suggestions as to what could be wrong really cause I don't want to spend another couple of hundred bucks and realize that wasn't the cause after all. That is if I can prevent it, I will try. Help please.