What important factors should I consider before purchasing a Wi-Fi repeater?

There are a couple of things you should be aware of, not all of these points may be applicable to you in your current setup & the new Wifi Repeater you intend to purchase:

• First things first, you need to know how the current WIfi signals in your house/apartment are propagating. If you have a laptop, use the following softwares below (free to use) and map our the signal strength in your apartment. Sometimes, just the correct placement does wonders.
• If you are in a single story house/apartment, try to locate your Wifi towards the center of the apartment, versus a corner.
• Wifi Mapper from Meraki (www.meraki.com) is great tool. You can get more information on it from: http://www.meraki.com/products/w...
• Another tool I recommend is the Meraki Wifi Stumbler - you can use it for discovering WiFis, use the following link: http://www.meraki.com/products/w...
• Understand and know which Channel you are using - it makes a whole lot of difference when you try alternative channels.
• Remember Wireless vs. Wired: Wifi operates half-duplex (it can only either transmit or receive at any given time), whilst the wired connection can do both at the same time.
• Make sure you don't have too many users. 4-10 users are fine for a regular AP/Router, after which most AP/Router for homes crap out.
• Make sure you have defined DNS services in your AP/Router, try using Google's DNS: 8.8.8.8 and 8.8.4.4. They can resolve a lot of issues.
• When purchasing a Wifi Repeater - see if you can invest in a Wifi unit itself. A Repeater has a singular function, but a Wifi AP can be used as a repeater and a stand-alone AP itself. It will be a few Dollars more, but can also serve as a backup.
• Do remember, then when you connect a Repeater, always do so in a Bridge Mode (else you will lose out on a lot of functionality).
• If you are in a multi-story dwelling, try to see if you can run an Ethernet between the different floors. It helps tremendously.
• Ethernet over PowerLines (Home Plug as Sandy McArthur suggested is a great solution). Its an added expense, but one that can really pay off. Now days, you can get a 200Mbps Powerline adapters for less than $100.
• It is always a good idea to have the antennas of your existing AP/Router and Repeater to be in the position as recommended by the OEM. Please do not try your own variations.
• Always avoid possible interference channels like the microwave, kitchen appliances, cordless telephone, etc. Try placing your AP Router/Repeater on its own away from walls, towards the center of your Room and House. The Wifi Stumbler and Mapper software are great tools in helping you resolve this issue.
• If you are a heavy user, invest in both an AP/Router and Repeater, one that is commercially more reliable. Cheap does not always necessarily translates to the best. Meraki, Ruckus Wireless and Cisco are great Wifi gear makers. There are many more, depending on your budget.