When booking flights when is it best to buy to get the cheapest flight.?

I am thinking of going to texas last week in oct, i have checked the flights this morning and its over €750 for flights only. when is it best time to book to get the cheapest deal? should i leave it to closer the time,
Answer
Well this flight from shannon, ireland is only €453 and seeing as your question mentions € i suppose you are in ireland. what airline were you trying? this is aerlingus.http://www.aerlingus.com/cgi-bin/obel01im1/bookonline/flightschedulelookupdispatchaction.do?bv_sessionid=@@@@0585808441.1181987756@@@@&bv_engineid=cccgaddlemjjkldcefecfigdffgdfkk.0or this one is leaving from knock, ireland using globespan and its only €439http://www.tradesecure.co.uk/flyglobespan/en2/euro/flightonlyta/atopweb.asp?wcu=newfltonly