Preston, themselves unbeaten in 11 league games, were the better of the two sides and should have picked up all three points to keep alive their automatic promotion prospects, but they could not convert any of the numerous chances they created on a frustrating evening.
The hosts almost opened the scoring with less than a minute on the clock after Mark Howard spilled Craig Davies' shot only to grab the ball at the second attempt at the feet of the lively Joe Garner.
The hosts continued to look the most likely to break the first-half deadlock and it was Garner again who almost opened the scoring on 20 minutes from Chris Humphrey's delightful cross. Humphrey whipped over a ball from the right, but Garner's downward header flew inches wide of the far post.
Bob Harris then had to be quick to block Garner's shot in the box after an incisive pass from Neil Kilkenny found the striker in a yard of space.
Conor Coady had the ball in the net for the Blades on the half-hour mark, but the goal was chalked off with the on-loan Liverpool midfielder a yard offside.
But Preston were soon on the front foot again with Garner, Bailey Wright and Paul Gallagher all denied after the Blades failed to clear a corner from the left. Wright's powerful shot was blocked by the hand of Michael Doyle in the box, but the referee waved away the hosts' penalty protests.
The visitors started the second half on top, but it was Preston who almost opened the scoring through Josh Brownhill. The youngster collected Craig Davies' knockdown only to see his effort rebound back off the upright.
And just minutes later Garner was again inches away from beating Howard. The striker got across the front of his marker to connect with a Humphrey cross, but this time his effort just evaded the far post with Howard well beaten.
Preston continued to press and Gallagher's header was held by Howard, but they could not break through a well-drilled United defence, who collected their eighth consecutive clean sheet.