In order to manage your labour resources effectively and keeping projects on schedule, from a General Contractor point of view, if you subcontract trades, you must keep a good base of subcontractors that you can work with, by keeping good relations and making payments on time. it is also necessary to have effective project managers in order to coordinate the work schedule. if you are a specialty contractor, you must have qualified supervision to manage your manpower effectively at all times, and have different sources of manpower in case of need arise, and make sure you do not take on too much work than you can handle. your resources could be overallocated if not managed effectively. this means, that you must have a good work schedule in hand for all your projects, which must be properly loaded with the resources you have, so that you can see at what points you will face resource shortage and then you can plan accordingly. again it all boils down to good planning and scheduling. this also implies that, you must know your scope of work well, in order to make a good schedule.