When choosing columns for nonclustered indexes, consider all the uses that were not satisfied by your clustered index choice. In addition, look at columns that can provide performance gains through index covering.
On data-only-locked tables, clustered indexes can perform index covering, since they have a leaf level above the data level.
On allpages-locked tables, noncovered range queries work well for clustered indexes, but may or may not be supported by nonclustered indexes, depending on the size of the range.
Consider using composite indexes to cover critical queries and to support less frequent queries:
The most critical queries should be able to perform point queries and matching scans.
Other queries should be able to perform nonmatching scans using the index, which avoids table scans.
