The most common reason for bad signal is objects in the line of sight such as metal, concrete which can lower the signal quality. If you can, position your router/laptop within a clear line of sight. Walls generally don't present too much of a problem unless they're thick. Metal and mirrors can also affect the signal.
Most routers use whats called an "omnidirectional antenna" which is a single verticle antenna which radiates the signal horizontally in all directions. The signal can become weaker if its not on the same level.
To improve this, you can use a "directional" antenna which is used to point and amplify the signal strength in one particular direction. This can be useful when you need the extra umphf to help with an interferring obstacle. This can of course be changed to point to the new location of your laptop.
Don't forget to look at the strength of the signal coming from your laptop as well. While adding a better antenna gain to the router may help the signal, you also need to check that the signal gian on your laptop isn't poor. If you have a PCMCIA cardbus wifi card in your laptop, then you can sometimes add a better antenna which will significantly improve signal quality.
