I agree with fermentation tips, but it is hard to say for sure. The only advice I feel confident giving you is to read some materials and do your best to UNDERSTAND what is happening, rather than just making it happen based off of instructions. I admit that in no way have I understand every little thing I do, but I am trying my best and it is really improving my beer quality.
What I mean is that you should try and figure out what exactly happens to the exact yeast you are using after you pitch it. Then you can determine what off flavors are attributable to what situation and you can then pinpoint the cause and work to eliminate it.
The other thing is to chalk up each failure as a learning experience after you learn to correct it. In my case my first two batched both had the same aftertaste I would call "bitter" and the second batch, a Kolsch, had a buttery tastes. I found out both of these are cause by too high of fermentation temperatures (I was in the mid 70s) or not enough time in the fermenter. I started the Tshirt/bucket of water around fermenter method, waited an extra two weeks once i reached my FG and the problems are gone.