There’s no secret formula for creating good tune sets. But you can use the sets that other people have played and recorded as a guide.
When you look up a tune you know on thesession.org, the first page (the Details tab) lists recordings of that tune. Click on an album title, and find the tune in the track list. Often, the tune is part of a set. If you also know the other tunes in the set, or if you can learn them, then you have a set that you can learn and play. There is no shame in learning entire sets from other people. And at least you know that the artist who recorded the album thought that those tunes fit well together.
For example, you mentioned Harvest Home. If you look up that title, you’ll see that it was recorded by Elizabeth Crotty on the album “Concertina Music from West Clare”. Click the link, and you’ll find that she played the tune in a set followed by the Liverpool Hornpipe. Maybe that’s a set you would enjoy playing, too!
A caveat about thesession: You have to be careful about the tune names. Often, a tune links lead you to a tune with the same title, but it’s really a different tune. However, with a bit of caution, you can find some good set ideas.
Another resource are the Foinn Seisiún books and CDs published by Comhaltas (CCE). The 2 volumes contain 220 tunes in 74 sets, all good session repertoire and well-matched sets. And the tunes are online!
First, to see an overview of the sets, go to the listings at thesession.org:
Volume 1: http://www.thesession.org/recordings/display/1227
Volume 2: http://www.thesession.org/recordings/display/1587
I know that these lists and links are mostly correct, because I worked with Jeremy at thesesion last year to fix them up.
Next go to the Foinn Seisiún pages:
http://comhaltas.ie/shop/detail/foinn_seisiun_cd_volume_1/
http://comhaltas.ie/shop/detail/foinn_seisiun_cd_volume_2/
Find the tune you want to play, and click the link. This takes you to the tune page, where you can listen to a recording of the tune. Now, at the right side of the page, under “Keywords For This Entry”, find the link for the set and click it. This takes to you to a set listing, with links back to the individual tunes.
For example, find “Boys of Bluehill” on the Volume 1 page, and click the link. On the tune page, click the “Boys of Bluehill Set” link under Keywords. Voilà, the complete set of “Boys of Bluehill, Cork Hornpipe, and Cronin’s Hornpipe” is displayed.
I’m sure other people will also have good suggestions, but these two should get you started. Good luck!