1) Buy what you need, not what the salesperson suggests
It may seem like a good deal to buy an all-in-one printer, scanner, fax and coffee machine, but if you never scan images and don't send faxes you will be buying technology that you will not use. Another problem with these all-in-one devices is that if there is a fault with the scanner you may not be able to print any more. If all you need is a printer, buy a printer.
2) Buy the right type of printer
If you are going to only ever print black and white documents on Letter or A4 sized paper, don't buy a colour printer. Rather, consider buying a B&W laser printer, especially if you are going to do a lot of printing, which makes a laser printer more cost effective. If you need colour prints or are not going to print regularly then an inkjet printer may be preferable, as the initial outlay is less.
If you are only intending to print photographs off from your digital camera look to see if the camera's manufacturer offers a printer designed specifically to work with their range of cameras. This may cost a little more, but the results will generally be better than what you would achieve using a cheaper inkjet on your computer - plus you will probably have the benefit of connecting the printer straight to your camera.
So, think about what type of printing you are actually going to do and buy the best tool for those jobs.
3) Check the price of consumables
Once you have decided what type of printer you want it is time to compare the cost of replacement ink or toner cartridges. Quite often the printer with the lower price will end up costing you more because the ink is more expensive. Remember, you only buy the printer once, you will buy ink or toner again and again.