If you've been using a good synthetic oil, you shouldn't need a flush.
If you've been using non-synthetic, a flush won't hurt and might do some good.
If you are going to do it yourself, I recommend using a flush made by AMSOIL or BG. Good products, inexpensive, won't break stuff loose. Have also seen good reports on a product called AutoRx. More expensive, takes longer to clean.
You could also have a profession machine flush, such as shops do using equipment made by BG and other companies---this can be considerably pricier.

As already recommended, a good synthetic will also clean, although more slowly than the methods above.