Keep him on his toes how? Why do you feel the need to play games with him? I'll bet this relationship won't last until the end of the year if this is your take on how a relationship should work. Pick up a copy of Glamor magazine (any one will do, there all the same) I'm sure there's an article in it called " 11 things to do to keep your guy on his toes"
To answer your question directly, you keep a man on his toes by playing games with his mind and heart.
Start small, by being unavailable and a little aloof, slightly detached and uninterested.
If the relationship progresses, you'll have to up the ante by randomly turning down dates, picking fights, being sullen and wounded at unpredictable times, and generally hard to get along with.
This will backfire in one of several ways, unfortunately.
A man with a shred of self respect will walk almost immediately. Those who stay will have less self respect.
Lack of self respect in a partner can manifest itself in various ways, ranging from being a panty-wearing push-over to full blown emotional and physical abuse. Whichever degree of instability you end up with is sure to be a roller coaster ride, and I don't mean the fun kind, so prepare yourself.
If you want to get yourself into a useable and sustainable relationship, instead of chain-jerking, try intelligence, respect, engaging conversation, outgoing and friendly attitude and understanding.
Your man won't be on his toes, but he'll be confident he's found the finest woman available, know that every man within eyeshot is envious and will fall over himself to keep you on his arm.