All the time.
Prefer working on one project at a time. Finish it before moving on to the next. The project should have a definitive start and end centered around clearly defined business objectives.
I don't mean to suggest you shouldn't have multiple projects in your work life. Just prefer to minimize the number within a context, say a team of people working together. That said, there is value in minimizing the number of teams you are involved with :)
A common issue in software development is run on development. This introduces many risks:
Constrain each project based on a set of business objectives. Keep the list short and projects will start and end quickly.
If higher priority objectives arise, decide if they can wait. Ask what the value of doing it now versus a few weeks or months from now. If there's really no value (other than maybe excitement) then wait! If not, then decide what to do with the current project.
In my experience, working on clearly defined projects one at a time means we spend less time on each project, we know when we are done and we can quickly move on to the next highest priority objective. This often negates the need to interrupt a project.
The first few discussions with a new consultant.
Ease into starting a project with new consultants. First invest in building some trust.
The first discussion is a great opportunity to talk about your business with the consultant. This understanding is invaluable and should be easy to talk about without advanced preparation. In subsequent discussions gradually introduce goals and problems.
Shared understanding is a great way to establish trust. Look for ways a consultant offers value through past experience, advice, and other feedback. This initial value can go a long way towards establishing trust.
Not to mention what it means for a consultant to invest the time to learn about your business and showing interest in what you do. Holding several discussions gives everyone a chance to reflect and bring even more to the table in subsequent discussions. This can be a quick way to weed out consultants that don't have the time to deliver quality solutions.
This is also an opportunity to show the consultant that you are willing to invest the time to build a quality solution.
Wouldn't your rather learn a bit about each other to see if the relationship will be a good fit?
The first few projects with a new consultant.
While taking on the risk of working with a new consultant, minimize other risks including time, complexity and low value. Nothing builds trust like a completed project that meets your objectives!