You can purchase or rent a fence stretcher or make your own. We used to stretch fence using 2x4s drilled in a way that bolts could be inserted through the fence between the 2x4s. The 2x4s need to be closed tightly to pull the fence because if you just pull with the bolts you will bend the wire and get an uneven pull. Then we would just use a come-along to tension the fence (or the pickup truck if acess was handy). Helps to have fence plyers as well to put some final tension on as you staple.
Be sure to X-brace corners and gates which will have considerable tension. Since you are using wood posts rather than T-posts you can also add a 2x4 upper and lower rails laid horizontally between the posts which can add tremendous stiffness to the fence when the wire is stapled to the rail. This is particularly good if you are doing horse fence or need to confine small animals. Its and inexpensive addition that keeps the fence straight and may even give you a place to sit.