2) Ask For Time.
If you know up front that there are a couple of offers on the table, or there is a high probability that you will receive another offer from a different company, just ask for time to make your decision on the solid offer that is on the table. It has been suggested that you email or call the hiring manager directly (especially if you have been dealing with an agency that is seeking to place you) and inform her/him that you need until a certain date to weigh all of your options before making a final decision. Additionally, inform the second potential employer you are interviewing with that you have a solid job offer on the table. This way they may be more inclined to speeding up the decision making process, especially if you are a viable candidate.