Most of the WiFi AP manufacturers also sell repeaters.
Plug one into an outlet halfway between your AP and your XBox, and, voila! See this .
Yes, to answer a comment, config is required to tell it which SSID to repeat.
gaming.stackexchange.com/users/None/
Have you looked into PowerLine networking?
Assuming your router and XBox are on the same mains circuit in your house, then that should be able to get a wired connection between your rooms without having to mess about laying new wiring.
It's also generally better bandwidth and latency than a wireless solution (though not quite up with proper wired ethernet kit).
lots of companies do it these days, a quick Amazon listing for Powerline networking kit has a whole list of them.
gaming.stackexchange.com/users/263/GAThrawn
I have the ASUS WL-330GE .
It can do several things, one of it is repeating/amplifying a wireless signal.
It works very good for me, although I do not use it as a simple repeater.
I am sharing WLAN with my neighbour and I use the router to make my own WLAN cell, using the WLAN coming from my neighbour.
All without cabling, all wireless.
Works great.
gaming.stackexchange.com/users/1669/Martin
I recommend reading some about DIY methods and antennas.
If You already have the connection, but it's too weak
I bet this will help: http://www.freeantennas.com/projects/template2/index.html
Just print it out on a thicker paper, cut&fold and stick some tin-foil to it.
Costs nothing and works!
gaming.stackexchange.com/users/3108/naugtur
Or, if you don't want to spend a lot of money, you can try using an aluminium foil or building your own cantenna .
gaming.stackexchange.com/users/2615/tunnuz
I have an XBox in my living room and my wireless router is in another room.  I can't lay a line so I use the XBox with WiFi. Problem is I drop my connection semi-frequently (very annoying when watching NetFlix or playing an online game.)    I'm not sure what causes the issue but I'm wondering if a WiFi amplifier would help, so my questions:   Is a WiFi amplifier a fair bet solution? Do they work?  What's a good, affordable one?
How do I improve my Xbox wifi signal?
gaming.stackexchange.com
<xbox-360>
gaming.stackexchange.com/questions/7542
gaming.stackexchange.com/users/1052/Nick Gotch
Try using a wired connection between rooms. You could go the wired connection route and connect them together.
User recommends using a wired connection of the two.