keep on your toes

3. PHRASE : V inflects
If you say that someone or something keeps you on your toes, you mean that they cause you to remain alert and ready for anything that might happen.

• His fiery campaign rhetoric has kept opposition parties on their toes for months.

... Cobuild

on one's toes: Fig. alert. (Typically: be / keep / stay ~ ; / keep one ~)

• You have to be on your toes if you want to be in this business.
• I have to stay on my toes to keep from getting fired.

keep somebody on their toes:
to force someone to continue giving all their attention and energy to what they are doing

• He gave me a couple of extra things to do just to keep me on my toes.

...http://idioms.thefreedictionary.com/toes