Hi there,
I used to get this when i first started brewing, and due to the hot Perth summers I was unable to keep a brew under 25c.
Then I got hold of a 100 can cooler from K-Mart and packed it with ice bottles, managed to keep the brew at a steady 18c. At the same time I bought a 2nd fermenter and started racking my beers. I keep primary going at 18c for 7-10 days and then rack to secondary at 18c for another 7-10 days. The improvement was good but still a slight "homebrew" taste lingered.
My next step was to look at the ingredients I was using. Standard Coopers kits with your typical kilo of dextrose. I had tasted good Coopers kits from mates so decided that perhaps it was the dextrose that was the outstanding issue. I started experimenting with using 500g dextrose/500g malt booster packs in place of a kilo of dextrose. The difference was notable, however still a bit of the taste lingered (although about 80% less than when I first started). It tasted like it was "amost there".
Looked again at the ingredients. I started trying the SAF yeasts and not the ones that came with the tin. The fermentation speed picked up a fair bit at the 18c temps than I was experiencing with the tin yeasts. Also, the sediment was fairly solid, quite compacted at the bottom of the fermenter.
Now it was tasting like good beer. I then read somewhere on AHB that it was preferable to boil the ingredients prior to throwing them all in the fermenter. So I began using an 11L pot to mix the can contents & booster packs to a slow/near boil for 10 minutes, letting it sit for an hour in the beerfridge and then adding it to the fermenter. Also when adding it to the fermenter I pour from around a metre high to maximise oxygen with the splashing etc and giving it a good stir.
At this stage the beer was fantastic. Friends of mine who were suspicious of homebrew were genuinely impressed.
Then I moved to kegging a couple of months ago. Even better, it tastes somewhat "cleaner" due to the lack of priming sugar required (my theory anyway). I still bottle 4-5 longnecks out of every brew (damn 18.9L kegs) which is good for beer mobility.
The difference between now and my first few brews is amazing. It now tastes better than any of the $30-40 cartons that I used to buy and the above improvements can be applied to any beer style, so I'm miles ahead as far as I'm concerned. The cost of a brew when I first started was generally $15 for a basic $10 kit and bag of dextrose. Granted, I do pay around $25-30 now for a brew ($10-$18 kit, $7-10 for booster pack and $3-4 for the SAF yeasts) but that is still $25-30 for over 2 cartons of very nice beer.
And the journey continues. I'm now experimenting with adding different hops to the boil and other things.
Sorry if I've gone on too long (this is almost a PistolPatch-length post
) but I thought I would share my experiences in getting rid of that homebrew taste.
To sum it all up I would say sanitisation, consistent 16-20c temps, lose the dextrose & use a booster, good yeast and evetually move to kegging (if desired).