Whenever you know that the column, or combination or columns, makes a unique key for a nonclustered index, I always recommend that you specify UNIQUE when you create the index. That information is great for the optimizer to have when generating query plans.

Think carefully about the impact of your clustering key on your nonclustered indexes. Any non-unique nonclustered index will be larger (more pages, eventually more levels) because the entire clustering key will be in the tree level.

If you have a clustering key with a column (or multiple columns) that are frequently modified, then those modifications have to be propagated to the tree levels of the nonclustered indexes as well.