What's the best strategy to deploy VLANs in an already running network?

Well it depends on your IT Budget, Your company geographical locations, How many depts you have, who should be isolated from whom etc. After all these considerations you will be going for the designing phase which may require new hardware. Just telling that there are150 users will not take us to the solution as obviously all those users are not under one roof or at same place. It will also depend on what infrastructure you currently have. After this you will go for the implementation phase and finally with verification and optimization