theres definantly a strategy to help your success rate but not gurantee it..
like teabagging said there are charts that are chosin from at random to upgrade for..
if you notice when upgrading to +6 if you burn 2 items then get a yellow.. the very next item is a 90% gurantee
or going +7 you burn 3 items its 80% chance and if you burn 4.. the next one is about 95% of making it.. i used these strategies and made +7 raptor.. along with 4 pieces shell 7 out of .. 1 raptor 5 shell pieces
i used paper armor as a replacement and burnt them.. btw you have to be going to the same upgrade point.. u cant burn +7's going +8 to make a +5
has to be burn going +5 so you can get a calculation on the % of making the next one, its like rolling dice and after each time it lands on something you erase it and your chances of landing on the right one increase a little.. not like flipping a coin. 50 / 50 all the time
edit: before i get flamed, this is also superstition based.. its my strategie and works awsome for me.. so wat eva floats ya boat..