1. If your passport has been issued within a year it is advisable to submit your old passport along with the current one. If your passport is older than 10 years, you would need to obtain a new passport to apply for a Schengen visa—have a look at the expiry date on the passport.
2. Make sure your passport is valid for at least 3 months after the visa expiry date. It should also contain two blank pages for any new visa stamps. In the case of you applying for a multiple entry visa your passport should be valid for at least a year or longer from the date your trip ends.
3. You can submit an application for most Schengen visas within 3 months(have a look on the specific country’s official website to make sure what they prefer) before your trip date. It doesn’t take that long anymore to receive a visa these days still it is advisable to submit your application at least two weeks before your journey starts(otherwise you might not have receive it in time if there is a hiccup in the system). However ask the advice of a travel agent, if you are not sure, because for some nationalities it may take between 3 and 4 weeks to process the visa application.
4. Make an appointment with the relative consulate or visa application centre well in advance. (If you need to have an appointment. Otherwise make sure your are at the consulate’s doorstep very early to avoid long queues.) During peak seasons it may come to a month waiting time for an available appointment. Rather make sure than missing it.
5. Probably the most important element of a visa appointment is having the correct documentation. It is also key to a successful visa application. I have seen many a sad travellers having to return home visa-less due to some paperwork being missing or not having the family members by their side that needed to be there in that specific situation. What you need to bring along: You must bring your passport (s), photos, visa application form, employment letter (or another form of employment confirmation), bank statements, hotel confirmation, flight confirmation, and travel insurance (as per the requirements for the specific country). If you are on a dependent visa, make sure you take your spouse passport and marriage certificate with you. Phone the consulate beforehand to make sure exactly what you need and who should come along. Ask them if they can tell you or maybe email you a list in what order they would prefer the documentation. Yes, they are sometimes fussy like that, but it makes a huge difference to their attitude towards you if you have everything exactly the way they prefer! Mostly there is a list pinned outside the embassy to tell you the order and you can organise it while waiting in the queue.
These days you have specific agencies that will do all the hard work for you. Find out if you have a agency in your area that will be able to assist you to save you some time and patience next time you have to obtain a visa. (See Gary Edwards’ comment below to get the website for Schegen visas.)