The expression 'to take the king's shilling, meant to sign up to join the army. Rather like with the 'prest' money for the 'impressed' man, a bonus payment of a shilling was offered to tempt lowly paid workers to leave their trade (an average daily wage during the Napoleonic period was 2p (at 12p to a shilling, this represented six days wages in one go). Once the shilling had been accepted, it was almost impossible to leave the army.
Since the army was not seen as an attractive career, recruiting sergeants often had to use less than honest methods to secure their 'prey', such as getting the recruitee drunk, slipping the shilling into his pocket and then hauling him before the magistrate the following morning (still hungover) to get him to accept the fact that he was now in the army. Sometimes the 'King's shilling' was hidden in the bottom of a pewter tankard (having drunk his pint, the unfortunate drinker found that he had unwittingly accepted the King's offer). As a result, some tankards were made with glass bottoms. Other recruits came from the courts, where a criminal's sentence could be commuted to service in the army – still the case (apparently) with the Blackwatch Regiment.
In fact the bounty for joining the army was much larger than a shilling. New recruits received £23.17s.6d, but out of this they were obliged to buy their uniform – a not inconsiderable expence.
Source: Richard Callaghan, Curator of the Redoubt Fortress Museum at Eastbourne, Sussex, on the BBC Education site