Hi Attila. My DC have just done SG and H (awaiting the results today!), and had some tutoring (what we could afford), but also we bought then the "HOW TO PASS...." book for each subject (How to pass standard grade chemistry/physics/maths etc etc). Amazon sells them cheap and has the full range. Only a few less popular subjects are not available.
These books contain everything your DC needs to know about each subject. They are really well set out too.
Past papers are available FREE to download from the SQA website (as I found out AFTER I'd bought them from Amazon!), and working through these is really valuable.
If you can, buy the course text book too (our school often shares the books between kids or photocopies bits, not much good for studying).
If you can find the money, a bit of targeted tutoring may be worth it - my DS2 (Dyslexic) had English and Maths tutoring for the whole of 4th year - one hour per week, per subject, and it made a huge difference to him. But it was tough to find the money at times. His Grades went from 4's to 2's in that time (his school just wasn't going to provide him with any extra provision despite his dyslexia).
I think there's a huge amount you can do for your DC's, while spending very little money (just the "how to pass" books), and I'd say the single most valuable thing you can do is do a little and often with them (or get them to do on their own), over the whole year. An hour a week on each subject (re-reading notes, going over the current topic), will do far more than a mad rush of cramming 3 or 4 weeks before the exams.
It is a bit of a drag at times, I admit, but I had both DS 1&2 tell me to "keep making me study or I won't"!