An even burn is really a factor of how the cigar was constructed. How experienced the roller is in constructing a cigar, the quality of the tobacco leaf he is using, his state of mind at the time, etc. are all factors. Once it is rolled, there is very little you can do, other than to keep it properly humidified. A dry, fast burning cigar will tend to burn unevenly quicker, exaggerating any imperfections. About the best you can do is to find a cigar that consistently burns evenly and add that to your preferred list.

PS: If you hear of or discover a trick, let me know. It's always fun to test another theory.