Know if you want a fixed or variable rate in advance.
Fixed rates give you piece of mind knowing how much you will have to pay whether you sign up for 2, 3, or 5 years. Alternatively variable rate is appealing at the minute because rates are so low. The danger is something could happen which results in them going up and suddenly you have to find extra money each month. Depends if you're a gambler I suppose.
Look if there are any over payment charges or limits. It sounds crazy to effectively give the bank more than they are wanting but it will save you thousands over the course of the mortgage.
The LTV ratios are important in terms of what rate you get and they go down in 5% increments. Eg It makes a massive difference if you make sure you're at say 85% rather than 86%.