Shylock loans money for interest (usance or usury). He obviously bases his interest rate upon the rates of the time because he mentions checking the rates when discussing a three-month long loan for Antonio in Act 1, Scene 3.
In general, in order for Shylock to be able to charge a decent amount of interest, he has to pay attention to not only what other usurers (moneylenders) are charging but also to whether Christians were lending out money without charging any interest. This is one of the reasons he does not like Antonio because customers would be unlikely to borrow money from Shylock at an interest rate favorable to him if Antonio and other Christians were lending the same amount of money with no interest.
In regards to Act 1, Scene 3, there is no specific method that Shylock uses to increase his interest. In fact, he ends up loaning the money to Antonio without charging interest. Of course, he has an ulterior motive, as seen in what Antonio's payment will be if he forfeits on the bond.