My top 5 reasons for booking your gap year early are:

1. Secure your place - many projects book up months ahead of time, some book up as far in advance as a year ahead of departure. If you leave your plans until the last minute, your chosen project may not be available.  
2. Spread the cost - The earlier you book the longer you have to raise the money and save for your gap year. By booking early you will generally only need to pay a small deposit (around £180) to hold your place, with the balance due much closer to departure.  
3. Secure any work permits you need - if you want to get a paid job overseas on your gap year you may need to get a work permit. For certain countries there may only be a small number of permits available which will run out quickly each year. As an example, for departures to Canada in November 2014, the work permits ran out in January 2014!  
4. Have something to look forward to - if you are still at school with one year left, the next 12 months are going to be the toughest in terms of education so far. By booking your gap year early, you'll have something to look forward to at the end of all of those exams and essays.   
5. Impress university admissions tutors when applying for deferred entry - by planning your gap year before you apply for university you are really going to impress the admissions tutors when they look at your application. They will see someone who is organised, motivated and will have had a worthwhile experience overseas. 

Don't delay! Organise your gap year over the summer and then focus on your final year of school, safe in the knowledge that your gap year is planned, ready and waiting for you at the end of it all.