Substitute Orange juice for lemon juice in equal amounts (1 part orange juice for every 1 part lemon juice).
Substitute white wine for lemon juice at a 1/2:1 ratio. 1/2 part white wine for every 1 part lemon juice.
Substitute white vinegar for lemon juice at 1/2:1 ratio. 1/2 part white vinegar for every 1 part lemon juice.
Substitute lemon extract for lemon juice at a 1/2:1 ratio and replace the rest with water. 1/2 part lemon extract and 1/2 part water for each 1 part lemon juice.
Substitute lemon zest at a 1/2:1 ratio and replace the rest with water. 1/2 part lemon zest and 1/2 part water for each 1 part lemon juice.
Substitute cream of tartar at a 1/2:1 ratio. 1/2 part cream of tartar for every 1 part lemon juice.