I love Christmas and collect many different items. I also put up more than one tree.
One of the things I do to change things up a bit is buy new ribbon (metallic, plaid, red) then also buy one other thing to tie it together. For example, one year I had red plaid ribbon and red poinsettias (I found inexpensive bushes that had about a dozen on each and cut them apart). I wrapped the ribbon through the tree, then when I am finished with ornaments, I placed the flowers throughout the tree, standing back frequently to balance overall. I have all my favorites, and for about $20-$30, the tree looks new as well! I have done it with gold ribbon and some gold balls that I found super cheap one year ($1 a box for a dozen glass gold balls).
I have spent as little as a few dollars, up to about $30, and because I have been doing this for years, I mix and match now, or come up with new ideas. One idea I have never had the time to do, but would like to try is to get the family to make paper origami cranes and use them as the theme. That is inexpensive and would be fun, if we can find the time!
One year I bought little glass lollipop ornaments, for a few dollars for each dozen. I used those and real candy canes for a candy tree. Another year I found glass icicles that were a dozen for around $3-4 dollars, spread though the tree they were really pretty and gave it an overall theme. I haven't used those in a while, so maybe this year I will look for some pretty sparkly silver ribbon and use them again . . .
One year, (along Rachel D's suggestion), I had a small tree in my daughters room that I decorated with her small stuffed animals, and it was one of her favorites.
I think the color theme works best if you place all your usual, sentimental ornaments on first, then choose something (anything you like!) last, so it stands out and you can make sure it is even over the tree.
I will also choose to highlight certain items, like putting Santa's more prominently one year, maybe snowmen the next. Grouping like things together gives them a bigger impact.
As soon as I get an ornament or decoration, it is sentimental, so I can't imagine getting new each year!
Not sure if you were just asking the question or looking for ideas, so I hope my answer helps!