A big plus would be to know Katakana as it is most used on signs etc. And that would probobly be extremely helpfull! Then Hiragana.

I could suggest that you make a list of common words and expretions that you could have in your pocket once you are there.
"How much does ... cost?"
"Where is the ... hotel?"
"When does the train arrive?"
"Where is the closest resturant?"
And of course the most important: "Where is the toilet?"

Just some examples, though I belive it would be rather usefull.

I recommend anyone getting a Japanese Phrasebook if they do plan on going over for a visit. It should have alot of phrases that should help you along like ordering food, using the post office, asking directions etc. It will show it in both Romanji and also Kana + Kanji along with the English

i stayed with a japanese family fer 5 weeks during the summer and they spoke no english at all and i only spoke a little japanese so wat i did was say a wor in japanese and point at the item and they understood that i wanted that or if i said a word and shook my head the would no that i did not like the item or i was allergic to it(and trust me im allergic to alot of stuff). if u try to develop a system with the people u meet in japan everything should turn out right they helped me alot so im sure people there will help u to

That could work if someone was with you all the time I guess...
to be honest.. it would be a little rude...

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