For the cheapest flights, travel mid-week rather than at weekends. (Tuesday, Wednesday and Thursday are usually cheapest. Monday and Friday are popular with business travellers. At weekends flights are busy with hoilday makers).
With Ryanair, some flights (usually to some of the less popular destinations), start off at mid-price then drop lower if there are very few bookings. So the cheapest prices are often charged a month or so before the flight date. Within the final week or two prices go up to their highest levels.
I've just tried a random example, based upon the principles in the previous paragraph, on the Ryanair website. I chose a less popular destination (Eindhoven) and selected mid-week dates about a month ahead. (Out Wed, 29 March. Back Wed 5 April). The fares quoted are �0.19 each way. (NB: This wasn't the result of hours searching. This was the first destination and combination of dates that I tried).
As Dzug has made clear, whenever you book, it's very hard to find real bargains to the most popular sunny destinations at peak times. If it's possible, try booking dates in March, May or September. Also be more adventurous in choosing your destination. Malmo, in Sweden, for example doesn't come to the top of everyone's holiday list but it's an interesting city with a good beach. A three-quarter-hour train journey will take you over the Oresund Bridge to Copenhagen, in Denmark, which is a vibrant city close to several attractive seaside resorts. I flew to Malmo, last September, for a day trip. The cost? �0.01 each way (before taxes of course!).
Chris