Use GZip Page Compression (You can edit that in the MyBB settings)
Look if there are any errors which may cause the slow speed.
Don't use a ton of plugins. Only use the ones you really need.
Turn on the debug information and look on which pages the forum loads slow.
Kys
the last one isnt really useful ? Well compressing the images to low size will also give you good loading speed. or else if you have alot of images! ge ta skin with no images!