Each project will be different, and you may also find that your potential employees have a variety of expectations.
For example, many folks may prefer a salary because they have a practical personal need to pay rent/mortgage and put food on their table. Especially if there is a need to hire them for a lot of hours (e.g., 40 hours per week for 6 months), your employees will probably be more comfortable with regular paycheques.
If the jobs are really small (e.g., less than a week's work), then paying on a per-project basis may work better. They may wish to get partial payment up front, with the remainder after the project is completed (and with larger projects there could also be a number of payments in between as the work progresses).
There may also some employees who are willing to agree to a "hybrid" of these two ideas, such as in the form of a base salary plus a percentage of the profits after the game is released ("profit sharing" which is similar to paying royalties, except that you'll probably want to be the owner of the work they've done for you). The risk is, of course, that the game never turns a profit and they don't get to realize the potential benefits of profit sharing (you'll want to make sure this is explained clearly in the contract -- the "Advice from professionals" section below recommends getting legal advice).
The positive side of this "profit sharing" approach is that it can provide a subtle long-term marketing advantage because those who get a share of the profits [as long as, or whenever, the game is profitable] will likely feel motivated to do "word of mouth marketing" for you by telling people "I contributed to that game; you should try it!"
Advice from professionals
Hire an accountant (a CGA designation is highly recommended) to make sure you get all the payroll and taxation aspects of your employment arrangements set up correctly.
Hire a lawyer (one with experience in both contract law and employment standards is preferred) to make sure your employment and/or profit sharing contracts are set up correctly.
For the profit sharing arrangements, you will probably need to have one short meeting with both your lawyer and your accountant at the same time.