I agree with Purlewise for both flat and round especially if there is a pattern to the knitting. I would like to add an alternate method for both because I really like to have options.
For a flat piece, I would divide the number of my sts roughly into thirds and add one stitch between the first third of the sts and the second third, then add the next st between the second third and last third. If there are 45 sts, then 45/3=15. Increase one stitch between the 15th and 16the sts and again between the 30th and 31st.
For a piece in the round, increase 1 st at the beginning of the round and one in the middle.