Simmer one tablespoon of cornstarch in 1/2 to 1 cup of water until it becomes transparent, stirring continuously.
I do NOT recommend microwaving this, unless you have an extra large container to do it in, and cook for only 30 seconds at a time, stirring often. There's really too much of a chance to boil over in there, and it does indeed somehow end up much hotter than on the stove.
This starch can be kept in the fridge between uses, but it is so easy to make up that I don't bother.
I dump my crocheted pieces in the starch while it is still quite hot and use a spoon to squish them all around. Carefully (hot- remember?) give them a bit of a squeeze, you want them thoroughly wet with the starch but not gloppy. It may take a few experiments before you come up with the right amount of starch to leave on. I have found that it is much easier to UNDER-starch, then if the piece isn't as stiff as you would like after it dries just repeat the process. If you over-starch, often the only remedy is to try to wash it all out and start over.
Pin piece flat to your blocking board or form 3-D pieces over an appropriate plastic covered mold. PLEASE make sure you use RUST-PROOF pins!!! I learned that the hard way and it is very discouraging to find rust marks on your beautiful pieces! It is SO worth it to invest in a package of good rust-proof pins- trust me on this! If there seems to be too much starch in the little nooks and crannies just use a good paper towel to dab the extra away.
Do let dry completely before unpinning. I often set my pieces in front of a fan to help speed up the process a bit, but drying does take time, so don't be in too much of a hurry.
This works as a refresher to any other method you have tried that hasn't worked quite how you wanted... just smooth some on with your fingers and let it dry.
I have been using this starch for years, and the earliest pieces done (like the little angel below which is about 25 years old now!) are still holding their shape extremely well with no yellowing occurring.
I have not ever experienced dulling with the cornstarch either, unless I mistakenly put too much on... years ago I used a cooked sugar water solution which I think does perhaps give a bit more shine but it takes longer to dry...
There's a lot of great information about other starching methods (and tons more info too!) on both the 'Snowflakes and Thread Crochet' and the 'Crochet Memories' websites.