In these rocky times when the first marriage is getting to be more like a trial marriage, one out of every seven people is likely to find aid and comfort in Susan Fields' warm advice for the second time around. Mostly -- you guessed it -- it's just the same ritual though it seems so much more serious (and Fields is at your shoulder to remind you that this time the blushes, sighs and giggles are as inappropriate as virgin white); only you're in charge of invitations, arrangements and finances while Mother, once you soothe any ruffled feathers, is purely decorative. Then there are the children, even if ""no less an authority than Amy Vanderbilt argues vehemently that your children should not even attend the wedding."" The formal ceremony is out; dancing is uncommon; the rice and garter are not thrown: the rule is don't overdo. Practical tips on flowers, gifts, vows (not to mention alimony), designed to cool a new kind of jitters -- and by the way, it says here that 90 percent of second marriages stay together. So don't worry.