If you have a cold, flying will probably make it worse - ideally you should delay the trip. Get a good night's sleep just prior to departure. There is much debate about whether it is better to fly eastward or westward. It may be largely a matter of personal preference, but there is some evidence that flying westwards causes less jet stress than flying eastwards. Again it is largely a matter of personal preference based on experience.
Most travellers think daytime flights cause less jet stress.
The dry air in aircraft causes dehydration. Drinking plenty of non-alcoholic fluids counters this. Water is better than coffee, tea and fruit juices. Alcohol not only is useless in combating dehydration, but has a markedly greater intoxicating effect when drunk in the rarefied atmosphere of an airliner than it does at ground level. Sleeping aids Blindfolds, ear plugs, neckrests and blow-up pillows are all useful in helping you get quality sleep while flying.