1. The most important tip that I can think of is plan ahead. I know it's not always possible especially if you are throwing a dinner party, but I honestly start planning for the parties I know about sometimes 6 months in advance. It gives me time to mull over the details, pick up items cheap that will work and craft things.
2. Keep a party journal or notebook. This is something that I have just started to do. Before I kept everything in a folder on my computer and pretty much forgot about it. Now I am using several pages to plan and document each party. This way I can take it with me when I shop and actually know what I am doing. I like to have a menu planning page, a decorations page and a crafting page. I clip inspiration from magazines and draw little pictures to remind me of what I want to do.
3. Pick a color scheme. It's easy to get overwhelmed when planning a party especially one with a theme. Sometimes you just start buying everything you see that is cowboy themed and you end up with a big mess. I find that picking a color scheme of two to three colors really ties things together. I also buy a lot of neutral dishes and serving pieces, white and clear glass, they go with everything and you won't need a second house to store all your party stuff in.
4. Think outside of the box for your theme. I don't think my kids have ever really had a character themed party, not that there is anything wrong with that. Sometimes though you can make a party more personal and special by using your a favorite color, book, food, music, candy, animal, toy, etc. as inspiration. I'm lucky because my kids usually pick the most off the wall party theme they can think of so I at least have a jumping off point.
6. This is the best tip, in the end parties are suppose to be fun. You shouldn't feel stressed and you shouldn't go over your budget. Do what you can and that will always be enough. Nobody is going to leave your party thinking to themselves, "Well, that would have been a nice party if she had just made one more fabric garland and a few more tissue paper pom poms". If they do, they're a jerk and you shouldn't invite them to your parties anyways. :)
Check out these great handmade party supplies from Louisville artists:
crazydaizies- party decorations and supplies