Everyone is talking about being aggressive when HU, which is fine, but it needs to make sense as well. For example, it is perfectly fine to raise pre flop with K high, every A high hand and so, however if the board is something like 7 T Q, and you missed, it is quite hard to C-bet as these cards could have easily hit your opponent. In lower stakes, don't bluff, like at all. You will get called down by all TP as well as mid pairs with decent kickers. When you consider that, don't be a hero and call other's raises with holdings like mid pair since it is not likely people will bluff often in lower stakes. It is too risky, and even if you do get it right a few times, it will cost you a lot more in the long run.
Also, value your made hands like sets, straights and better. With those, you can probably call lighter then you would in some tough spots in MTTs.
As an example, I suggest watching Doug Polk vs Phil Hellmuth Jr. in heads up match (King of the Hill HU tournament). There is a hand where DP called with a straight and pretty much knew PHJr had a flush. If that was a regular MTT with 9 people at the table, that would be a fold so many times.