If the covering letter seems hellish, it's nothing compared to the particular torture that is the synopsis. I have heard agents say brightly, 'Oh, I never read them, it might spoil the story.' To which one can only answer 'Why ask for them then?' before running them through with an unsharpened toasting fork. Because ask for them they do. So, as a writer desperately seeking representation, you will have to resign yourself to condensing all those months and years of hard work into a page or two of pithy prose.
First things first. Remove the toasting fork with a twist, then shove it straight back in, because there's no consensus among agents as to exactly what they want from a synopsis. One page or two, or ten? Single or double spaced? To include character breakdowns (to possibly accompany your own) or not? Look up the details for each agent you're sending sample chapters to check if they have any particular demands. If nothing stated, shorter is better than longer. One side of A4 is usually enough, maximum two pages, spaced as you wish but in a clear font such as Times New Roman in 12pt. Whatever length and spacing you go for, fill each page - the ones I've seen that go over to two sides, but only by one paragraph look as if you either ran out of steam or lost confidence in your writing.
Stick to the main characters - having workshopped lots of synopses I know that people get confused if there are many more than four names, I'd say a maximum of six before most readers lose the plot (literally). If pushed, use generic names for minor characters - waitress, chauffeur, teacher, children. Try a few telling character details: a leather arm chair of a man, a cool blonde with an eye to the main chance, rock n roll anarchist.
Pin point the genre. If in doubt, where will it be shelved in Waterstones? If still not sure or going for 'fiction', then who do you write like? Then go and look where they're shelved in Waterstones. That's your genre. One thing I can guarantee is that you haven't come up with a whole new genre. Crossover is a cop out. Now think about the theme - coming of age, redemption, the worm turns. Write a sentence on the theme. Now the plot - bored housewife takes series of lovers to escape humdrum life in provincial France. You might need a couple of sentences for this.
Tired? And we're still on the opening paragraph. We'll look at the rest tomorrow.