This thread consists of twelve things that everyone who's looking into solar power for robots needs to know but that I haven't seen anywhere.
Note: this "tutorial" assumes you know what a solar panel is and the difference between a panel and a cell along with having a basic understanding of the math behind electronics.
1. The most important thing you need to know is solar panels are weak.
Solar panels produce a ridiculously small amount of power (forget about fitting a 10W panel onto a robot, it's just too big.
2. Whatever the panel is rated for is about twice what you will realistically get. The most you can hope for from an 8V panel is around four to five volts under direct sunlight.
3. Solar panel power directly correlates to size. You already know this but what you may not have thought of is the fact that you need a bigger robot to hold this panel. Bigger robots are heavier. Heavier robots need more power to move and thus a cycle appears. You will always have to make the panel bigger than the chassis otherwise it just won't work.
4. Don't be cheap. Four separate panels will never produce the same power as one panel of the same surface area simply because of the wasted space on the edges. Also remember that you get what you pay for.
5. DC/DC converters are your best friend as they are the only way to reliably get a large amount of power out of a panel. Don't think you can buy a 0.5V panel and step it up to 6V though. You'll lose all of your power to inefficiency. The best thing to to is get way more panel than you need and step it down. For example say you needed [email protected]
for whatever. You would need to buy a 18-24V panel which would give you a solid [email protected]
(that's the ballpark you're looking at for a large panel). Using a DC/DC converter you can turn that power into [email protected]
because your input and output needs were both 6W. So I guess number five has another message: make sure you don't try and pull 8W out of a 2W panel. Physics says it just won't work.
6. How you angle the panels at the sun makes all the difference. A few degrees off can be the difference between full power and what you get on a cloudy day. Because of this fact horizontally mounted solar panels will do best at noon and worse in the morning and evening.
7. If you go with several cells as opposed to one panel you'll notice that the power reduces drastically when even one of them gets shaded. Using a single panel and converter can help out with this 
8. Keep the cells/panel cool either via airflow or water. The sun is hot and it'll make your solar panels hot too.
9. Mount them in such a way that vibration will not be an issue as glass can break very easily (duh
) There is a way around this:
10. There are two kinds of panels: ones made of glass and ones made of epoxy. Always get the epoxy ones if you can because of two things: they don't shatter and you can drill them for mounting. Epoxy ones also don't seem to get as hot but that's a major 
10 and a half: There is another school of thought where the panel is stationary and the robot simply goes over to it to get a charge every once in a while. Personally I think it's not that much harder to make it chassis mounted but you do whatever you want.
11. You can use mirrors and lenses to increase the efficiency of the panel but this will add serious weight, cost, and complexity to the project as you will
need cooling if you do this.
12. There are all kinds of solar cells/panels out there so shop around. The difference in power can be tenfold from one to another. (The cost will usually be tenfold too
As usual, let me know if you think of something else or if something isn't correct.
[6-12 thanks to soeren]