Not a bad question. It's interesting and worth a try.
The Shettles method is completely debunked- don't bother with that. Chinese gender charts are fun but not based on anything concrete.
Go and look at genderdreaming.com It's a fascinating site which sets out theories behind their recommendations, and the suggestions also get tweaked according to the success various group members have. Essentially it follows what someone posted above, which is that more girls are born in a famine. The theory as I last read it was that high nutrition, optimum health, high fertility, robust weight, bigger muscles all set you more for a boy. Almost everything that lowers fertility also makes you more likely to have a girl. The typical "girl mum" described on the site is thin, either does a lot of cardio and no weights, or no exercise at all, picks at small amounts of low nutrition food, so a handful of crisps and half a cheese sandwich and "too busy to eat", unlike the typical "boy mum" who takes down three solid meals of salmon, lentils, wholemeal bread, root vegetables, high fat dairy etc. and lifts heavy weights. Statistically older parents have more girls, which ties in with the low fertility point. Obviously skinny women can have boys and weightlifting ones can have girls- its just about moving your chances a bit in one direction. In terms of timing of sex etc they have some more complex suggestions which seem to factor in masturbation in the days before as well as ovulation timing. The site makes it clear that even a perfect "sway" only moves your chances a few percent, say from 50% to 70% which doesn't mean it doesn't work, just that it's not expected to work for each individual even if overall it works for the group. It made logical sense to me as it's known that girl foetuses are stronger so able to withstand poorer conditions, whereas boy ones being more frail need all the advantages to make it.
You are also very very slightly more likely to have a boy anyway because more boys are born.