To summon a particular Hero, Masters either need an abundant source of mana, or something to negate the need for so much magical energy. The method behind the first option is usually the mana-filled jewels or relics that Karma mentions in her first post. The amount of mana required varies from Hero to Hero; as Karma said it has to do with the renown of the Hero, how old their legend is, and how powerful they were reputed to be. Summoning Joan of Arc, for instance, would be more difficult than summoning Doc Holliday; partly because Joan of Arc is more renowned than Doc, partly because her legend has been around for longer, and partly because she is perceived as having more spiritual power (being chosen by God). Contrastly, summoning Loki is greatly more difficult than any of them, having existed outside the human concept of time, and having the power of a god himself.
The method behind the second is to use a catalyst, an item with a significant sympathetic connection with the intended Servant; that is, something that was close to the Hero during their life. This is ideal when the Master has very little or no magical power of his own, or when the prospective Servant is normally outside the Master's abilities to summon. As an example, being a mage of next-to-no aptitude Shirou was only able to summon this particular figure
as Saber because he unknowingly possessed this
at the time. Catalysts can also be used unintentionally, as demonstrated by Rin summoning this particular figure
as Archer, using a mana-charged gem that actually served as a catalyst.
When compared to the other Servants portrayed in FSN, the ones chosen by people here would need much less mana, seeing as their stories comparatively don't date back too far. The exceptions to this would be Loki (a much older legend) and Joan of Arc (a Hero of exceptional spiritual power). Whoever chooses these Servants for their Master character would more than likely need to do a little background research into their Hero, and decide on a catalyst to use. Calling Loki, for instance, could require a powerful relic from Norse mythology, such as Mjolnir. Joan of Arc could be summoned using one of her old letters, or a standard she bore into battle, or the arrow that pierced her neck at the Battle of Les Tourelles.