We got engaged in 2009, and started planning in October, 2010 for our wedding this december - so 2 and a bit years in advance!
At first we just booked the major things like church, cars, venue, and photographer, as these can get booked up very far in advance where we live.
I started looking for dresses at Easter 2011, and ordered it last October - be aware it can take 6 months for dresses to arrive from the point of order.
I think you are wise to start planning the major things now, and then you have plenty of time to browse for the less important things such as invites and favors. This is what I have done and I have got exactly what I wanted, often at discounted prices in the Christmas sales over the past 2 years!!