Depending on the length of your intended hitch, consider writing the name of your destination followed by “の方面”- “direction of". This ensures that even if drivers aren’t going to your end destination, they might get you half way there. Alternatively, choose a destination that is a bit closer than where you are aiming for to write on your sign, as more people are likely to be going there.
Smile a lot. It is quite intimidating for some drivers to pick you up, so try to look as friendly as possible.
Lawsons or any store like that always have a stash of cardboard that they will let you have a piece of to make a sign.
Even more useful are the little white or blackboards which you can pick up at any 100yen shop, along with a marker pen or chalk, and which most drivers find really cute!
While you're there, most 100yen stores also sell an incredibly useful map of Kyushu, which can be used for planning routes and locating challenges.
It is ILLEGAL to hitchhike while standing on the actual expressway, or the immediate road leading up to it. Generally, once you get to the expressway, it is best to walk a hundred metres or so away from the lead up road to somewhere where you and your sign can clearly be seen, and from where drivers can clearly see there is somewhere to stop. Depending on the expressway entrance you use, there is a good chance that you may be able to stand out of view from the meanies that work at the toll booths and hitch OK.
There are no two ways about it - hitching out of a big city is a nightmare. You are allowed to use public transport within cities to get to within walking distances of the entrance to the expressway. Of course, you can also use national roads, which run through all cities. Journeys obviously take longer though. One trick I have found works quite well is to go to somewhere like Lawsons and as people get to their cars, say “Excuse me, how far is it to the expressway/where is it?” etc. When they answer, always ask how long it takes to walk there. Put the stress on the fact that you are walking! Nine times out of ten, they will be surprised, ask why you are going there without a car, and, after you have explained what you are doing, they will offer to drive you up there at least as far as the expressway entrance.
Think about carrying sweets, cans of drink - anything you can think of that you can offer drivers and their families/friends. Some small omiyage from your home town or ken can also help to break the ice - I took stickers and badges donated by Trinita, the Oita pro football team, on my last hitch, which went down well and gave us something to talk about!
You will probably be offered a lot of business cards, so make sure you have something to put them in so you don’t have to stuff them into your pocket. If you haven't got any of your own yet, you may want to consider investing in some - it's only polite to return a card, and you may make some longterm friendships on the hitch!
Always offer to buy drinks, meals etc for people at service areas. They will nearly always refuse and pay for you, but it is nice to offer.
I think that it is the hitchers' responsibility to make conversation, as the driver has been good enough to pick you up. It is a great chance to use your Japanese. There is no need to worry about being embarrassed by mistakes, as you will probably never see those people again. Just try everything you know!
I recommend bringing/buying/borrowing/renting a tent and some sleeping bags, as the weather should be fine for camping, and hotels/youth hostels may be tricky as it is peak travel season. Of course, if you are lucky, then people will offer for you to stay at their houses. If not, and it is getting dark, simply ask them if they know anywhere good for camping.
Finally, this is Japan, and we all know how safe our new home country is - but that doesn't mean you should leave all common sense at home. Be careful when hitching after dark, and whilst 99.9% of people will be trustworthy, keep your wits about you.
All in all, have fun, meet people, practice your Japanese, and go get hitched!!!