Teams can delay free agency for top prospects by keeping them in the minor leagues for the start of the season and ensuring they obtain less than a full year of service time (172 days) in 2012. The appropriate timing of the player's debut depends on whether he has a 40-man roster spot.
Prospects such as Shelby Miller and Mike Montgomery who aren't on their teams' 40-man rosters must spend less than 172 days in the Major Leagues this year or they'll be on track for an early arrival on the free agent market (post-2017 vs. post-2018). The 2012 season will last 182 days, so prospects not on their team's 40-man roster can safely be called up after April 17th or so (I'm including a couple of buffer days).
Prospects like Bryce Harper with 40-man roster spots must be optioned to the minors for the beginning of the season and spend at least 20 days on optional assignment. These players won't end up a few days short of a full year of service; they'll either spend the full year in the Major Leagues or end up 20-plus days short. As long as 40-man players such as Harper aren't called up before the end of April, they'll be on track for free agency after 2018.
It's always important to keep in mind that service time is just one element of a team's decision-making process. Plus, a substantial proportion of top prospects get optioned to the minor leagues early in their careers. The player's readiness and the team's needs generally trump service time considerations, but there's no denying they're a factor.
I recently explained the timing of prospects' debuts in this piece.