When considering a low magic campaign in which the players still have access to spellcasters, you really have to consider nerfing the spellcasters back a ways.
We've already seen someone mention limiting the levels they can achieve. Forcing them, in effect, to multiclass.
You could also consider some of the following:
significantly cutting back the spell slots they get, maybe stop each level at 2 or 3 instead of 5
pruning the spell lists available per class to keep to a less flashy or powerful set of options, maybe emphasize fellow party member buffs like bless, magic weapon and other spells that help out in fights indirectly rather than directly
increase all casting times to full actions or 1 round casting times