Transactions should be divided up into small increments to reduce the risk of losing money.

Say I wanted to trade in $50 worth of BTC for $50, but instead of exchanging it all at once I would do it in 10 small transactions of $5 each. So one by one I make the $5 exchanges, check to see if a transaction is completed, then I move on to the next $5, and then the next until I exchanged all $50 worth of BTC.

That way if the service ends up scamming me I would only lose $5 worth of BTC instead of all $50 of it, then I would mark the service as untrustworthy and take the $45 left of BTC I have left and try another service.

If this is possible to do within a reasonable amount of time then it's a good way to risk only a fraction of the amount you're exchanging.