First, you need to figure out where the increases go. Suppose you have 98 needles in work and the pattern says increase 13 stitches evenly across the row.
Compute the approximate placement of the decreases using the following formula:
(current # stitches - # to increase) / # to increase
(98 - 13) / 13
85 / 13
6.53 rounded down = 6
This tells us we need to space our increases approximately 6 stitches apart.
Determine where to start based on whether you're increasing by an even or odd number of stitches. If the number of increases is even, position the two center-most increases on either side of center. E.g., if the distance between increases is 6 stitches, the two center-most increases would be on needles 4L and 4R, leaving 6 stitches (3L to 3R) in between.
If the number of increases is odd, position the center increase on either side of center (1L or 1R).
Determine the placement of the other increases by counting off needles working from the center towards the edges. For this example, you would leave 6 stitches between each increase. Temporarily mark the location of each increases by pulling those needles to hold.
Use a garter bar or decker comb to move stitches right or left starting at the edge. You may have to make several passes if your decker comb is not wide enough to transfer all of the stitches at once.