And make sure the source .psd files are "layered" and not "flattened". Have each element of the design put onto a seperate layer. Having your designer work in multiple layers gives you more flexability later on.

Get a written and detailed project plan, payment schedule and a signed contract. Do not do verbal or email contracts, make sure they are printed and signed by all parties.

Do not pay more than 50% deposit on the total price. I would suggest 25-30% upon signing of the contract/agreement. And then additional payments based on milestones completed. Do not make the final payment until you are satisfied with the project and have copies of all artfiles created.

To save money you might try and negotiate a non-exclusive design. Hereby the designer can sell additional versions of the design to other customers.

On the other hand, if you are paying for a custom design then you should have exclusive rights to all content created, and the designer should have no rights to sell to anyone else. If this is the case the project should be a "work for hire" contract so that you own the copyright to final product.

The biggest trap we fell into with our early design work was upcharges associated with "changes" to the approved and signed contract of the work order. I cannot emphasis how important it is to do a very detailed and through pre-design needs analysis. The time spent ahead of the project can save you time and lots of money on the back-end. Do not assume anything, be very detailed when writing your contract regarding what you expect and include specific timeframes.

There is an old Israeli saying; "A discussion now is an arguement later on." The more detailed and specific you are upfront the better the outcome.