Andrew Strauss' England side needs only to draw the five-match series to retain the Ashes, but Prior says England learned its lesson about celebrating too early after its setback in the fourth Ashes test last year.
Australia fought back to level the series 1-1 with victory at Headingley in 2009 before England recovered to win at The Oval and reclaim the urn.
Prior said on Thursday that England is looking no further ahead than its three-day game against Victoria at the Melbourne Cricket Ground starting on Friday.
"You've got to be careful you don't fall into the trap of too much back-patting too early," Prior said. We've played well, the tour's gone well so far, but it's all in the past now."
The third Ashes Test starts in Perth on December 16.
Prior, who has batted only twice in the first two Tests and averages 27 in the series, will play as a batsman only against Victoria with back-up wicketkeeper Steven Davies taking over.
England will field a new-look pace attack against Victoria with Stuart Broad ruled out of the tour after breaking down with an abdominal strain in Adelaide.
Chris Tremlett, Tim Bresnan and Ajmal Shahzad, who have nine tests among them, are in line to replace Broad.
Prior said all three were capable of doing a good job for England in the third test, although he admitted Tremlett was a "like-for-like" replacement for the tall Broad.
"There are obvious likenesses with the two, mainly with the height and bounce they get and the areas they bowl," Prior said.
"Tremlett with his height and bounce can be very awkward at times. When he gets it right he's very hard work."
Victoria has named four players to make their first-class debuts in the three-day match, including teenager Alex Keath, who had an unbeaten 192 in a recent under-23 game.