I live in Connecticut, hardiness zone 6a. Last Summer, fescue grass took over a large swath of the sunniest part of my rye grass lawn when it was very dry. Most of it died over the Winter and we reseeded very successfully in Spring. However, some of seems to be sneaking back in where the new rye grass didn't take so well. What should I do to keep the fescue from taking over again this year?
Let it take over.
Here in Iowa (hardiness zone 5a) most lawns are mixes of bluegrass, perennial ryegrass, tall fescue and creeping (aka red or chewings) fescue. If you get the correct varieties of the fescues you have to get down on your hands and knees and study each blade of grass to see the differences between these four turfgrass species.
Most commercial prepackaged grass seeds are not purely one species/variety. They are a mix of species and varieties (hopefully) suitable for your area. The advantage of this is that each species will predominate where it is best suited for the microclimate in that exact spot. In your case the tall fescues will probably predominate in the dryer, sunnier and higher traffic locations, and the ryegrasses will predominate in the shadier, wetter areas.
My recommendation to you would be to reseed or overseed with a high quality mixed species grass seed mix. That will establish a better looking tall fescue variety than your wild/weed variety. Then, if needed, hand pull any ugly looking grass that snuck in.