I have spent much time thinking on this "how does one become a trophy wife" question. For years I have been toting the "first marriage is for money, second marriage is for love" mantra, and so am always on the look out for rich old men. I have found that the man-luring techniques are going to change for both the age class and tax bracket you're aiming for, so there's no clear, easy answer.
There are three general classes of men who are looking for trophy wifes:
(1) For some men a significantly younger woman is all they need. Unfortunately these men can be hard to live with and put up with as they have very low expectations for themselves. They also fall along the tax bracket continuum, rather than being pooled at either the high or the low end of the wealth scale, so make sure you see their portfolio and bank statements before pre-nups are signed. These men are recommended for women with confidence issues and/or a dire and immediate financial situation; and as a warning, they can sometimes be scared off by strong personalities. However, if you're willing to go there, I suggest finding someone quite old with a pre-existing medical condition.
(2) On the other end of the spectrum, some men will take only outrageously young and ridiculous attractive women with a D-cup minimum. These guys are usually high-maintenance and aren't that desperately lonely because they have high expectations and know they can be filled. Because they fall in the top tier of rich old men, they have the financial security to be picky. Education level can be a limiting factor with these men. If you're in grad school, for example, one would assume you can at least read 3-syllable words, so these guys might not go for you. I'm not trying to be insulting to you and/or your physical appearance, but it's just not going to work based on your intelligence. They will fear your reading ability and the likelihood of you tricking them up somewhere in the pre-nup.
(3) The last general category of man are those just looking for a young, decently attractive companion and is willing to invest in a good personality. These men can be rather nice and doting as they don't have as high expectations, but can form respect. General observations indicate that these are often self-made men with a bit more respect for where they came from, but have developed a trophy-wife fetish over the decades from hanging out with men with trophy wife fetishes. Apparently its a communicable disease. However, these last men are usually pushing the bottom limits of the "trophy wife tax bracket".
(4) I know I said there were three general classes, but this class of man is not rich, he's the dirty old man. Many trophy wife hunters will be dirty old men, to be sure, but they are rich dirty old man. This is a huge distinction as many things in life are more tolerable with wealth. These men will disguise themselves in expensive suits (pay attention to their attire, because often they will rewear the same suit or two suites and just changes shirts and ties - a dead give-away), surround themselves by other rich men to whom they can be snobbish, and try to woo you with bank credit. This is why it is so important to get personal, visual confirmation of their financial assets before entering into any legal agreements. Just one of these men could lead to the down-fall of a perfectly acceptable future trophy wife.
So, its really going to depend on what you're looking for in an old man. As for me, I'm not really attractive enough to really be a trophy wife, so I figure I need to find a really old and visually impaired man - let me know if you can set me up with anyone, and I'll return the favor.