1. Taking ideas from other types of juggling
This already happens a lot anyway, but when you consider the different types of juggling and all the styles within it, there are a nearly an infinite range of tricks that can be done. That doesn't necessarily mean doing a trick exactly the same with another prop, you can take an idea and try it, and maybe due to limitations or the type of prop you use find another way to do it. In doing so you most likely come up with something you would never imagine normally.
Examples:
Toss/contact
Rings/clubs
Cigar boxes/stacks? (I'm playing with this now...)
2. Taking ideas from dance.
Getting inspiration from the movement of the body and the shapes created through dance is another good choice. Choose any type of dance you can think of and find a video on Youtube. Pause it at any point and see if you can find a way into or out of that position using your props in the process. It's simple but surprisingly effective. Of course there is also the option to do a style of dance while juggling at the same time!
Examples:
This is how different contact can look with a tutting influence (Noiseassault!)
If you want really out there stuff watch Storyboard P for some weird influence.
3. Creating shapes
Take the tricks you can do and try to create a certain shape out of them, be it a square, circle, or whatever else you can think of. Then try to think of ways to going into that shape again but in a different body position. Creating this start and end point for yourself is limiting, but makes you think in a different way than you usually would and quite often leads to new ideas.
Examples:
Yamamura Yuri (3 ball triangle shape)
Some guy (Contact style triangle shape)
http://youtu.be/rXUzWu730xc?t=23m47s
4. Using colour
This is something I've been thinking about more and more recently. Having a two coloured object or a single prop of one colour and a lot of props in another makes you think about ways of differentiating the colours, either by revealing them at certain moments or or finding new patterns that contrast/compliment the colours used. From the perspective of the viewer it looks really effective too.
Examples:
This scene from a Japanese skate video actually set me off on this...
Some guy again (sorry...)
5. +1
+1 basically means adding an extra ball or whatever prop you are using to the tricks you are doing. This is really simple but I would often not consider it for a lot of tricks, and then when I did I realised it created something new or interesting that I didn't expect (and sometimes things that are too hard to do...) This doesn't necessary mean just sticking it in the middle of the pattern to make it harder either, just the fact that you have this extra thing in your hand might mean the other method you always used will not work any more, so some sort of change is needed.
6. Mixing props
Again, this is another thing I have spent a bit of time on (particularly with RdL rings). Thinking about ways to use the other prop to throw/roll/balance on, or in the case of rings go through/around, you get a whole new range of pathways that open up which aren't otherwise possible. You can also combine two types of props and do both individual styles at the same time. Then there are bridges, new shapes etc. Go mental!
How about mixing a contact ball with thuds?
Another example (again, sorry...but it fits the topic!)
7. Putting limitations on yourself
I find limiting myself in some way a really effective way of making routines. Doing it is really easy too, any kind of limitation is fine. Examples would be only using a certain part of your body/not allowing yourself to use your hands/always keeping a prop in contact with your body a certain way etc. I could go on forever on this one, but I'll leave the ideas to you! Try to make a routine of a set length (1 or 2 minutes) in order to force yourself to keep coming up with new things instead of giving up after getting stuck. It's usually those points that lead to an interesting breakthrough!
8. Using sound
The sound of catches/making noise with your body/hitting the props against yourself or the floor is also another direction to explore. I haven't tried this personally but there are plenty of people who have!
Examples:
Wes Peden
9. Modifying your props
This might sound like a strange one, but by attaching something to your prop, be it another prop or something like a magnet or string, it completely changes the dynamic of the object and allows for completely new tricks and routines.
Examples:
10. Using the world around you
Who says you just have to stand there and completely disregard everything around you? There are no rules, so just look at all the paths/objects/gaps/hole etc. around you and fit it into your routine! A particular recommendation is going to a playground with climbing frames/various fun objects you can go in/around/on. Even if it's stupid, who cares?
11. Using your clothes
This is something I got from watching Wes's Synthetic video. Using something with a zip/hood/something baggy gives you extra places to roll or stall the objects you use and thus leads to tricks that otherwise aren't possible. This isn't really explored that much outside of the use of hats so there's bound to be a ton of undiscovered stuff still to be found.
Examples:
http://youtu.be/rXUzWu730xc?t=11m29s
12. Buying a ball/club/ring shaped object not meant for juggling
If you look around at toys for children/pets you can sometimes find some really interesting things that will also work as props for juggling.
A few good examples:
Babal ball (Thanks RdL!)
Hol-ee Roller (Lots of uses for the holes, plus if you press down on them with an object it will shoot it up quite a decent height. Expect a video soon...)
13. Look outside of juggling
Pretty much anything is ok here, whatever gets you thinking creatively. I tend to get some good ideas from skate videos, there are some really creative videos out there amongst all the generic stuff.
Examples:
Gou Miyagi
William Spencer
Daewon Song
14. If all else fails, find another person
Doing things with a new person basically opens up a whole new world of possibilities, even for the most basic of tricks due to the many different ways you can share/pass props between you. And should that ever get boring you can apply the above to get even more ideas...
Examples:
Wes & Patrik
Hopefully that's enough to keep you busy for the next few hundred years! If not, here's some more.
Jay Gilligan
RdL at JJF 2012
Comaba 1&2