Take a look at how the big engineering consultants work. Every time they get a project, they hire. When its over, they end the contracts.
There is usually a good pool of self employed contract engineers out there so when you need a skill, you hire it. You may pay more per diem but you only need them for the duration of the contract. They look after their own tax etc.
Now it depends on what skills you need. It may be an idea to try and find out just how available and from what agencies you get these skills. If you need to impart special skills you may need to employ, train and retain.