Over at JPF I answered by suggesting doubling your own vocal, i.e. singing it twice (if you can get it the same or close to the same). You'd be amazed at how many of the hits you've heard over the years have two or more layered vocal takes.

Also, consider the judicious use of chorusing. One or two chorused tracks buried under the main vocal can add substance.

the Insolent Lad
Enough small empty boxes thrown into a big empty box fill it full. ~Carl Sandburg