Slower Spell Return 

Magicians have it hard in a S&S campaign. Spells are more fatiguing to cast and spell slots may, at the Dm’s discretion take longer to return.

One Spell Slot Per day. In this variant the sorcerer gets back only one used spell slot per day, starting with the lowest level one. This simulates those worlds where magic is enormously fatiguing and magicians cast very few spells. This has the effect of making them consider very seriously whether to use them in the first place.

Conditional Spell Slot Return. In this variant sorcerers get back their spell slots only when they fulfil a condition or it is fulfilled for them. Performing a sacrifice to a Demon God, or resting for 8 hours in the throne of skulls deep within their tower of power is an example of the former. Getting back their spell slots once per month when the moon is full is an example of the latter. For maximum pain to the sorcerer these two options can be combined.

Rare Magic Items 

Magic items are very rare in Sword and Sorcery campaigns. To simulate this you may want to ban most item creation feats. Alternatively you can make the XP cost for magic item creation equal to the material cost.

Ability Based Casting 

Using this variant you do not need to use spell slots instead you can simply cast any spell you know and make a spell fatigue roll as described above. When exhausted you can still cast spells by overdrawing on your power. If you fail your spell fatigue roll when overdrawing you take Constitution damage equal to the level of the spell +1. Once you have taken Constitution damage this way you cannot cast any more spells until the Constitution damage heals. Creatures who do not have Constitution scores overdraw on their Wisdom score instead.

Using this variant you can add metamagic effects by adding to the spells level when making the spell fatigue roll and calculating ability damage. 

Limited Lists 

The other way to limit magic is by limiting available spells. In classic S&S very few direct damage spells should be allowed. GM may wish to ban them all. 

The main forms of offense should be summonings, limited monster summonings and mind spells.

Banned Spells 

Here is a list of spells the DM should consider banning. 

Comprehend Languages. This makes deciphering ancient documents just to easy in a campaign where knowledge is power.

Read Magic. Once again this spell makes the deciphering of magical texts too easy and just does not fit the atmosphere of S&S games.

You might want to consider banning the really gross attack spells such as meteor swarm. They just don’t really fit the style of the genre. In some campaigns the DM might want to consider banning all direct damage spells.