Doing it yourself (or DIY) decorating is always a consideration for the adventurous person. It is not a consideration your want to let get out of hand when there are professionals who will not mess your project up. Things like kitchen and bath upgrades will usually require a professional in the end because of safety and building codes. This is, of course, unless you are a plumber or an electrician yourself.
Structural work on your home will require an architect or builder and a licensed contractor. You cannot get around that either. The do it yourself enthusiast should concentrate on projects that do not require construction, plumbing, or electrical work. Doing your own decorating can save you money in the end. You also have control over the job schedule and you do not have to invite strangers over. What you save in money, you will expend twice over in energy and time. There is a reason people pay professionals to do this. For that reason, some people prefer to split the job. If you like to sew but you are a messy painter, make your own upholstery and curtains while the painter you hire recolors the room.
Interior designers are great with aesthetic choices but they do not have an exact image in their head of what you want. They can, however, draw floor plans and construction documents. If they have ASID on their business card, they have a degree in this stuff. This means that they can be used part time as advisers to save you money, but you can do the shopping and work yourself.
A designer will charge in a variety of ways. Some designers use a flat fee to cover both their work and supervision. Some charge a retainer and get you with an hourly rate. These are the best to use as advisors since you control how much time you need them. Watch out because some designers add a percentage to the materials they purchase for you with the discount they recieve at professional shops. As with contractors you will want to take bids and collect references. Interview designers and view completed projects if possible. Make sure to view their portfolio and get a feel for their personal style as well. You want a designer that listens to what you really want and who is willing to provide only the amount of service you need.
The best way to decide how to do a job and how much help you need is first decided by how a room should function and feel.