Oil in coolant - 1998 Honda Accord 2.3 litre V-tec 4 cyl

My vehicle (with 109,000 miles on it) over heated and I found excessive amounts of oil in the cooling system. I sought the advice of two auto mechanics and they both felt it was a bad head gasket. I've been a shade tree mechanic for years, and felt I could handle the job with the right repair info. I purchased the repair manual published by Honda Motor Co. And procceeded to do the job. I thought I had suceeded and patted myself on the back but soon realized I had the same problem again on a smaller scale. The only things I can say that I didn't do by the book were (1) I used an aftermarket gasket set instead of genuine Honda parts and (2) I did not have the head planed ( the head appeared to be within the allowable warpage limits of.002 inch ). Did I do a bad job or could the problem be caused by something other than a head gasket. I'm not sure how long the oil had been accumulating in the system before it over heated the first time I discovered it. Any insight on this will be greatly appreciated. Thanks!