Schools minister David Laws said the cost of clothing was often "unnecessarily high" at a time when family budgets were being squeezed.
An Office of Fair Trading investigation last year suggested that three quarters of schools placed restrictions on where uniforms could be bought.
That typically added £5 to the price tag for each item, leaving parents an estimated £52 million a year worse off.
Speaking as the Liberal Democrat conference gets under way in Glasgow, Mr Laws announced new guidance for schools.
Exclusive single supplier contracts should not be used, unless regular tendering processes are run to ensure firms provide value for money.
They should also not enter into 'cashback' agreements with shops.
Compulsory items of uniform should be available relatively cheaply, and branding should be kept "to a minimum", under the guidelines.
Schools are urged to avoid changing specifications frequently.
Mr Laws said: "Costs at the start of a school term can quickly add up, particularly for families with several school age children. "School uniforms can be an important sign of identity and pride, but at a time when many family budgets are squeezed parents should not be forced to spend more than they need to.
"I want to see fewer schools using single suppliers and branded items, which keep costs unnecessarily high."