Learning to cut your own dog's nails not only saves money, but it can alleviate a lot of stress from your dog.  I will discuss how to buy a pair of nail clippers for your dog.  I will discuss how to buy a pair of nail clippers for your dog.
For a large dog, the nail clippers should be sturdy enough to cut the nails without excessive force.  I prefer not to use the "guillotine" clippers that have a loop at the end, for a wiggly dog, this just a hard, extra step.  I prefer to use clippers that resemble trimming/pruning shears.  They are heavy duty and easy to control.
For small dogs, you should use a pair that is not too large.  I prefer to use "scissor" like clippers.  They are smaller and aren't as harsh on the small nails as the bigger ones used on large dogs.
Find a pair that fits your dog's size and also is comfortable for you to use.  eBay is a great place to look.  I would recommend only buying a pair that is still in original packaging to cut down on germs and fungus, etc.