The absolute FIRST thing you should be doing when planning a wedding is setting up your budget and allocating costs to each item. Do some research to find average costs for each item, and make sure you include absolutely everything, from the venue down to the bombonaires. Once you've seen it all laid up, you can start to look at where you might be able to reduce costs if needed. Laying it all up like this also means you're not going to be surprised halfway through the process by a big bill, and you know exactly what you need to be saving to pay for your big day.
Sometimes, using a wedding planner can actually help you save costs (I know, sounds crazy right?) - BUT planners know all the pitfalls and surprise costs (after all, we've done it so many times), and we often have supplier rates in place with some preferred suppliers so can save you a few dollars along the way.