Maybe this is too obvious, but I've seen a lot of new sailors do it.
When reefing the main, you don't just tie/hook the reef clew to the outhaul. You must also take a turn around the boom. This can be done with the outhaul itself, or as I like to do, a short piece of webbing, like a sail tie can be run through the reef clew and tied around the boom, independent of the outhaul. Snug this strap up enough to have the reef clew as close to the boom as possible and yet leave it loose enough so you don't crush the crap out of the gathered sail cloth in that part of the sail. A little give in this strap/line, will also allow for adjusting the outhaul.
If you don't rig this line, your boom can hang down several inches from the reef clew. Also, you are putting tension on your small reefing grommets along the body of the sail. This messes up the shape of the reefed sail and these small reef grommets are only for gathering up the part of the sail that isn't being used, along the boom. There should be little or no tension on them. Adjust the outhaul to take said tension.
Also, as mentioned by others, be sure, after you hook up the reef tack, to pull up the halyard, nice and tight. In high winds, The luff of the sail should be straight and tight. If it is scalloped between the slugs, (or there are a lot of wrinkles forming perpendicular to the luff up the sail, in the case of a bolt rope sail) the halyard needs to be tightened.
I find that in very strong winds, you sometimes need to re-tighten the halyard after sailing awhile, as the sails bolt rope and the halyard itself will tend to stretch some in strong breezes. A handy way to do this is with a cunningham, if your sail has the grommet to rig one. I have a floating goose neck that rides in the sail track with a 4:1 purchase to pull it down (just like the boom vang, but is rigged from the goose neck, straight down to the bottom of the mast). This works great. Instead of having to try to uncleat, tighten and then re-cleat the halyard, at the mast, I just give this purchase a pull from the companionway.
I'm no authority on all this, so maybe Judy would comment on whether I'm on the right track here
I also was wondering (For Judy again) if re-stretching a bolt rope is a good idea
(by hanging the luff between two points with a come along at the tack and cranking on the luff some, after unstitching the bolt rope at one end, of course) I've done it before with good results, particularly on a factory 26X mainsail. It seems that (for awhile anyway) that the 26X factory mains had this shrinking bolt rope problem a lot.
Best Breezes,
Steve K.
Mac 26D
, Highlander. That's so obvious I overlooked it 
.