One of the advantages in getting a sub-contractor to work on your project is the overall reduction in costs.
Before hiring one, make sure to do your basic research. Don't forget to look at references, qualifications, skills, quality standards and accreditation. One tip: ask if a sub-contractor is licensed. If they are not licensed, you will most likely get a better price. The reputation of a good sub-contractor travels by word of mouth, so do ask around.
Have a set of questions on hand, depending on your requirements (fees, hours and availability, equipment), plus other important matters based on your particular situation (is the sub-contractor willing to work evenings and weekends? Does the sub-contractor have medical insurance?).
Before interviewing prospective sub-contractors, make sure you have all the plans of your project outlined from start to finish. Having a detailed budget is crucial, as well. It will give the sub-contractor an overall view of the scope and limitations of the project, and most importantly, you wouldn't want to run out of funds in the middle of the work, would you?
One vital thing to remember when dealing with sub-contractors is the sub-contractor agreement. It doesn't have to be a huge deal involving an attorney and detailed legalese. The sub-contractor's agreement simply outlines the expectations between you and the sub-contractor, the objectives, the scope of the project, and the method of payment.
Putting together a simple sub-contractor's agreement protects both you and the sub-contractor, and saves you the trouble of having to deal with business relationships ruined by verbal agreements gone sour.
Ideally, it would be wise to manage and supervise the sub-contractor's work, check the evidence of work done and make sure that contractual obligations are met. However, if you cannot be on-site the whole time, make sure that part of the sub-contractor's agreement states that the job site be kept clean and safe from hazards the whole time. This is to protect yourself if you are using unlicensed sub-contractors, as you are assuming liability for them while they are working on your project.
If you plan on using sub-contractors on a regular basis, consider setting up a database of contractors and sub-contractors that you know and trust.