About five years ago I was introduced to a handy travel planning tool: the Wardrobe Grid. It's really very simple — I do mine in Excel, but it can be done on paper too.
In brief, potential travel garments are entered into the grid on the X and Y axes. Everything that hangs from your shoulders goes on the X axis (coat, jackets, dresses, tops — one item per column) and everything that hangs from your waist goes on the Y axis (pants, skirts — one item per row). Each item on the X axis gets a number as well (since some of these things can be worn together); if you are in the habit of wearing skirts over pants then your Y axis items will need numbers too... ;-)
Then... if something on the X axis works well with something on the Y axis, colour in the square where they intersect. If tops 8 or 11 work well with a particular jacket and pants, write "8" and "11" in the square where the jacket and pants intersect as well. And so on. If a combination needs a particular pair of shoes or whatever, note that too.
Pretty soon, after charting how your clothes work together, you'll have an excellent idea of which garments will give you the greatest number of options and combinations. You'll also be able to spot the things that you won't wear so much because they only go with one other garment or require shoes that you won't be able to wear with anything else.
All this is very handy when you're packing to ensure that you get the maximum number of possible outfits from the minimum number of garments.
Actually, such a grid on a larger scale is a very useful way of analysing your wardrobe generally — seeing which clothes are giving you best value and where the redundancies and "holes" are. But I mainly use it when planning what I'll pack for a journey.
The best instance of it working for me was when I moved to the States to work a few years ago. My main wardrobe, books, furniture etc. were going to take two or more months to arrive and there was a limit to what I could take with me on the plane. By making a wardrobe grid and choosing clothes that I could combine in lots of different ways, I was able to get by at work for six weeks without feeling I was wearing the same thing every week. Towards the end of that time I made some comment about being glad when my other clothes would arrive, 'cause I was starting to get bored, and my boss commented that she hadn't even noticed that I was working from a limited wardrobe!