During the Roman Republic the army was a citizen militia manned through conscription. That means that most service was not done on a volunteer basis, but rather because as a Roman man it was your duty to report to the levy (dilectus) until you had served the minimum required number of campaigns (due to a manuscript issue, historians debate whether this was six or sixteen years). That said, men were probably often given exemptions for a variety of reasons (recent service, being over the age of 35, etc.). And clearly there were men who had fulfilled their campaign requirements and technically did not need to serve, who nonetheless might present themselves at the levy, basically as volunteers. The centurion Spurius Ligustinus seems to have done this as his career progressed well beyond the minimum campaign requirement. In a few instances, consuls accepted volunteers in lieu of conducting a politically unpopular levy (e.g. Scipio in 205 BC, Marius in 105 BC).

That said, the police mechanism for the levy seem to have been relatively limited: it rested on men being willing to register for the census and then show up for the levy. And we know that there were grinding and unpopular wars that men simply didn't want to fight. We also know that there was great enthusiasm for some wars that promised considerable loot, which saw people flood the levy. And in any given levy there were people who clearly pushed to the front and said "pick me!", while others either begged for an exemption (granted by the consuls and 10 per tribune of the plebs), or simply hung in the back sulking and hoped the legions filled up first.

Importantly, no one was officially promised land. There was no enlistment contract as this is for modern volunteers that might specify certain bonuses. Prior to 177 BC, there was a good chance that once you served as a veteran you could join one of the colonies that were being founded at a pretty brisk clip on land taken from the Italians who defected during the Hannibal War. That colonization came to a halt in 177 BC for reasons still somewhat obscure. After that there was clearly a good deal of land hunger, hence the land distribution schemes of the Gracchi brothers that were popular with Roman voters. From Marius on we see generals seemingly making political promises to get land for their veterans, often using their own veterans as voters to push through these policies by flooding the polling spaces. The main reason Pompey the Great joined the First Triumvirate was because he was struggling to get land for his veterans from the east; Caesar ultimately carried a bill to provide them with land in Campania.

But again, none of this land was guaranteed, and fights over land distributions contributed to the bitter divisions that destroyed the Republic, as well as toxic alliances between generals, tribunes and the veterans themselves to secure these distributions. Augustus, keen to preserve his autocratic position and maintain the loyalty of the army, ultimately established a retirement system for the new standing army, that provided a 12,000 HS donative to legionaries who served for 20 years (initially 16, but lengthened due to lack of funds); this was the first time that Roman soldiers had a guaranteed retirement bonus.