Usually I actively use only 10-20% of menu items, especially in different kinds of catalogs, so I wonder if there in any practice that highlights the menu items that are most chosen by a certain user. For example slightly change a background or font.
MS Office had adaptive menus for a few years, where the least used items would stop being shown without additional action by the user when the menu is open. It never seemed to please many people, as it kept the menu list unstable for scanning and position memorization.
Moreover, deciding when to put something into the category of 'most used' and when to remove it is tough, as you don't want items close to the threshold bouncing in and out of the most-used category over the course of say a day. It's something that sounds easy, but in practice isn't.
It seems that a person who is always hitting the same items may or may not gain some efficiency because their most used items are presented a differently, but they will certainly wonder why their menus change unless you do the additional work of working an explanation into the experience as well.
The BBC website used to have a feature where the various sections on the home page (sports, world, business, etc) most frequently accessed would shift in color and emphasis. It got quietly disabled some time ago, and now another redesign is in progress.
I disagree though that having different styling for specific menu choices isn't going to have an effect on selection speed, especially if there are a large number of items to scan through.
Changing the background colour or font (as you suggest) is probably the simplest way of doing it.
I would go with changing the background colour in a subtle way rather than the font. Also I would use a different colour for each of the highlighted items (and keep it consistent). That way you combine the colour, highlight, position, and text to help people find the menu item faster. Maybe something like this:
You could add an icon, but that may cause some confusion, so I would avoid it.
What you shouldn't do is change the order of the items, or remove / hide some items. That causes confusion and messes with people's positional memory when selecting a menu item.
I would rather not implement any "automagic" highlighting since (as others have mentioned already) this tends to confuse users.
A good approach, in my opinion (aside from revamping the site's structure so that users rarely encounter unscannable amounts of items) would be a "starring"-type system that lets users mark their own "favorites". Gmail's inbox is a rather good example of how it's done right.
There also might be decent "secondary" approaches, such as stackexchange's very own "favorite tags" solution: Users can select a "kind of stuff" they are interested in and it gets more or less subtly highlighted from there on.
Most important point, in my opinion, is that the user has to stay in control of what happens at all times. Microsoft's "adaptive menus" have been mentioned above; I witnessed people literally smacking random stuff into their monitors because of the permanent frustration caused by menu items disappearing.