convention for using sequences

Rookie question, I know, but I googled around and couldn't find a
quick answer ...

How do I get the next value in a sequence? I've noticed Rails'
migrations use a single-row entry table to store the current version
(SchemaInfo I think it's called). Now, I know I can simulate this by
having a single-column table containing just the ID [ by deleting all
rows in the table, then creating a new one and getting it's ID, but
this seems horribly laborious and very un-rails like). But is there a
convention for doing this? Some sort of nice little single-line I can
stick in the model or something?