Maximize the RAM: 2GB* for the May '06 (Core Duo) or 4GB** in the Late '06 (Core 2 Duo). The RAM mentioned should be in a kit; 2GB means 2 each 1GB chips and 4GB means 2 each 2GB chips.
Also, make sure that you have ample free space on your HDD. Figure at least 10-15% free. If it's more filled than that, free up some space or replace the drive with a bigger one. While you're at it, make the replacement a faster (7200rpm) one.
* Do not attempt to put more than 2GB of RAM in a Core Duo or it will not boot.
** By putting 4GB into your late '06 MacBook, your machine will only use 3GB but will have an advantage over a 1GB+2GB configuration.