The speedy former England Under-21 striker added to Dave Kitson's earlier opener with a superb strike and sealed victory with a stunning solo effort late on after David Healy had pulled one back.
Both sides had chances to break the deadlock before Kitson's 11th-minute opener.
Kitson himself flicked a header wide and volleyed over the crossbar in the opening three minutes, while at the other end Eddie Lewis was denied by the legs of fellow American international Marcus Hahnemann in the Reading goal.
But Kitson finally found his touch to break the deadlock with his third goal of the season - all of which have come at home.
Paul Brooker's deep centre was fumbled by Preston keeper Andy Lonergan and Forster helped the ball into the path of Kitson, who made no mistake with a low drive.
Lewis had an immediate chance to reply, but was again denied by Hahnemann as the keeper parried his weak side-foot finish.
The midfielder's misses were punished by another striker's finish on 34 minutes, when Forster held off Youl Mawene to sweep home another Brooker pass.
Preston hauled themselves back on the stroke of half-time when Healy thumped home a free-kick from the edge of the area after Richard Cresswell had been fouled.
But instead of bringing the game to life, the goal had the opposite affect as the game degenerated into a scrappy second-half affair.
Preston had appeals for a penalty waved away when Lewis tumbled under a challenge from Graeme Murty and Cresswell flashed a header wide from a late Lewis cross that looked certain to produce a goal.
Lonergan had earlier kept Preston in it with a stunning save to deny Kitson, but Forster had the final word with a stunning winner five minutes from time.
The striker picked the ball up in his own half and beat off the challenge of three defenders before squeezing an angled low shot past the powerless Lonergan to move Reading up to fifth.
Reading: Hahnemann, Murty, Williams, Ingimarsson, Shorey, Hughes (Convey 74), Sidwell, Harper, Brooker, Kitson, Forster.
Subs Not Used: Sonko, Goater, Newman, Young.
Goals: Kitson 11, Forster 34, 85.
Preston: Lonergan, Mawene, Curtis, Lucketti, Alexander, McKenna (Skora 64), Lewis, Etuhu, O'Neil, Healy, Cresswell.
Subs Not Used: Daley, Smith, Lynch, Ward.
Booked: Alexander, Mawene.
Goals: Healy 45.
Att: 11,857
Ref: R Beeby (Northamptonshire).