You can use this module with the following in your ~/.xmonad/xmonad.hs:

 import XMonad.Layout.LayoutHints

Then edit your layoutHook by adding the LayoutHints layout modifier to some layout:

 myLayouts = layoutHints (Tall 1 (3/100) (1/2))  ||| Full ||| etc..
 main = xmonad defaultConfig { layoutHook = myLayouts }

For more detailed instructions on editing the layoutHook see:

XMonad.Doc.Extending