I have read that creatine has a short helf-life in the body, so if you want the strength gains do it before you work out. You might be able to get an extra rep or two if you are doing stuff in about the 8-12 rep range. It won't increase your 1RM.
Acid breaks down creatine so the juice thing prolly isn't a good idea. Almost all commercial preparations of grape juice contain ascorbic acid as a preservative.
If you REALLY want to send that stuff into your muscles, take 5-10 grams on an empty stomach and then follow with dextrose about 5-15 minutes later. However I doubt that the extra hassle is worth it, just drink the creatine together with the dextrose. It'll also help it go down easier.
PS for future reference this question belongs on the supplements board