I'll tell you what my experience has been with doing the ground school, and give you some tips that I found useful, and maybe the combination will be of some use to you.
TIP ONE - buy the PPL Confuser and sleep with it under your pillow. It's the best £20 I ever spent, BUT don't think you can get through the exams just by memorising the answers, you can't. Sometimes they sneak similar but tricky ones it, and you wont know as much about flying as you should. Having said that, if you've read the books (Thom is good) and you can answer all the questions in the Confuser for each topic and get them right, AND have read all the answers as revision, then you'll "fly" (ahem) through the exams.
AIR LAW - Do this first. I nearly lost the will to live because it got so boring, but it's important to get out the way early - particularly as some schools wont let you solo until you've got it. Some bits are useful to remember, but most you can look up subsequently if necessary. This one you can cram for, and use the Confuser.
HUMAN PERFORMANCE - quite easy and largely common sense. You can cram for this, so do it early. This will take you half the time to read through and practice using the Confuser as Air Law, and is quite interesting. TIP : read through the Confuser questions BEFORE you read the Thom book. This will give you a clue as to what you should concentrate on, i.e. the Thom book has a load of (quite interesting) waffle about anatomy, but you don't need to know much of it to answer the exam questions. ANOTHER TIP : remember the difference beween the symptoms of Hypoxi and those of Monoxide poisoning.
WRITTEN RT - sadly this one is not represented in the Confuser. Use the CAP 413 book, not Thom. This has some useful examples in it. The exam is fairly short and the CAP 413 doesn't take too long to learn. I did this one early and before I'd used the radio much in actual flying, but you may find it easier to do later on once you'e used to using RT in practice. TIP : learn the order of the May-Day and Pan-Pan calls parrot-fashion. Also learn the order of position reports etc. ANOTHER TIP : know the content of a radio call in Special VFR flights (there's a page in the 413).
METEOROLOGY - this one was my favourite and is very interesting. You can't really cram for this as you need to understand the concepts and practice analysing the 214 and 215 weather reports, plus you need to learn the TAF/METAR abbreviations. Take your time over this one and use the Confuser religiously, especially practicing the 214/215 bits.
AIRCRAFT TECHNICAL - no doubt about it, this one's a PIG. Not so much because it's difficult, although some of the concepts take a while to get your head completely around, but also because there's a lot of detail that you do need to know. Again you can't cram this one because you need to understand the concepts/prinipcles and take the time to understand and remember the detail. Same ritual with reading, doing the questions in the back, doing the Confuser questions and reading through the answers. Try to get through ALL the Confuser questions say 3 times before you do the exam. It will pay big dividends - not least speeding up your response time in the exam, when time is of the essence.
AIRCRAFT PERFORMANCE - even though this is a fairly small one, do this one AFTER Aircraft Technical, as you need to know/understand some of the concepts from the Technical section. You can cram this one to some extent, but you need to do it AFTER the Technical one, which you can't cram. Not horrendously difficult, but you'll need to memorise various things, such as the ratios for take-off distances etc. Confuser very helpful again!
NAVIGATION - quite fun actually, but do this one last. I had done several nav trips before I took this exam and had already been taught by my instructor to user the Flight Computer and various other techniques, so I found it quite easy. For this reason, it's good to do it last, because you will already be practiced at using the techniques which will save you time in the exam, and trust me, you'll need it. The Thom book is big and is a pain in the butt - we used a manual written by David Cockburn which was a lot smaller, had all the pertinent facts and some useful practice exercises (but does have some mistakes in the answers). The sections on course correction and 1/60 rule were very useful. Having said that, it was useful to still have the Thom book to look up the odd thing. The Fabulous Confuser was particularly useful for this exam.
I was working full time when I did these exams, so was only able to study at night and during the weekend. If you want to cram some exams before going to the US I would do Air Law, Human Performance and possibly Written RT. If you're brave, you could try the Met as well. The others I would take your time over. I took about a month each to read throug the Thom books, do the practice questions at the back, do all the Confuser questions and read through the answers 3 times, then do the exam.
I got 90% for the Air Law (because I hadn't twigged that I should read the Confuser First), 95% for the RT (because I hadn't remember the page about Special VFR) and 100% for each of the rest of them - so my method works!
That was a bit more of a long winded response that I was anticipating, but I hope you find it useful. Doing the exams is a pain in the butt, but if you do it right, you wont have to do them again!
Good luck, and have fun in the States. And don't Buzz any cows.
