I'm not very good at elk either. With mule deer I do really well. On the old elk101 site there was a contest for guessing elk scores. You can look at those and try to guess a little bit. You can probably google some kind of a scoring game I would guess. That is about the best way I think. Or everytime you go into a sporting good store try to guess the size of the elk and then ask the owner what it was. They will probably know.
I try to do as much field judging as I can. I also like to go over to my brother-in-law's taxidermy shop and measure up some of the antlers to see how close I can come. We have had some good bets on scoring. Over the last few years I have gotten better, just because I have seen more bone up close.
I was told by a very expirenced elk hunter about field judging bulls on the hoof. He told me that if I think I'm looking at a 350" bull, to start with a base score of 200. That 200 comes from an average spread of 40", 30" of mass per side, and 50" main beams per side. 40+60+100=200. Then all you have to do is add up the tine length. I have used this before and have been fairly close, however practice makes perfect. Hope this helps.
I read recently that the average length of a bulls body (from shoulders to hind end) is 50 inches long and height (from top of back to belly) is 30 inches. If their beams are as long their body with really good tine length and width you would be looking at a good 350" bull. I think I read it in the latest Eastman's Bowhunting Journal I got.
Another good resource I've read was out of Eastman's Elk Hunting the West, the guide starts on page 241 and goes through pg 251. It includes pictures as well as some good body size references for quick and relatively accurate field judging.