I try to stick to things that I will actually use, or at least display, as so many times in the past I have bought T-shirts that I never wear, key rings I never use (my set of keys is already too crowded!), ornaments I have no room for and other bits and bobs that end up in drawers!
So now I go for things like pens, notepads and other stationary that I can use at work or around the house e.g. I got these really cute Disney post-it notes from DLP last trip! They are also useful for decorating scrapbooks/photo-albums with anecdotes etc! :-D. Also photo frames and photo albums themselves make nice souvenirs!
I like mugs and other kitchenware that can be used regularly (e.g. tea towels, cutlery, utensils, crockery, oven gloves, placemats, even pet bowls!).
I also collect magnets since they are easy to display in the kitchen and are nice reminders of particular trips if you get ones with the year printed on. On my last WDW trip I actually found a triple whammy - a pretty magnet that is also a clip (e.g. to hold leaflets, cards, paperwork etc) with the year printed on it! It is currently stuck on my boiler holding my dentist appointment card :-D
I did have a Mickey shaped toothbrush holder in the bathroom from the WDW Tower of Terror gift shop which I loved but it sadly got broken! But they have tons of lovely/useful bathroom accessories in there!
I think calendars make another useful souvenir while also acting as a print/poster! I love my DLP one featuring a different attraction for each month
Biscuits that come in a souvenir tin you can take to work, then keep the tin to use for anything at all! I have one to keep spare pin backs in and another to keep batteries in etc.
Finally, I collect pins; I usually buy attraction pins, park specific pins, and always look for one with the year on to commemorate each trip. (I also trade from my ebay pin-lot for any other pins that take my fancy). I display all of my pins on a board at home so I look at them frequently (again, I don't want them sitting unloved in a drawer!). The reason I don't collect vinylmations is that wouldn't have the space for them! Plus another reason I like pins is that I can keep the magic alive a little throughout the year by trading whenever I find myself in a city with a
Disney Store that still does pin-trading in the UK
I'm sure there are many other things I've forgotten, but basically I just look for things that I can get some kind of use out of! ;-)
. I like to get T-Shirts with different characters every time I go. 
