Though no translation is perfect, there are ways to make yours better than others that are out there. One obvious way is to do your homework and use all the right terms. This will take you a long way in your career as a legal translator. However, unless you thoroughly proofread your work, you are prone to sending less-than-polished work to the client. If the client notices and complains about the mistakes, you are going to have to correct them and apologize and hope they continue to regard you as their favorite translator.
Here are some proofreading steps that work for me:
Any time you decide to use the plural in the TL text when it is singular in the SL text, make a note of it. You can use the highlighter in Word or make a note if you are using a CAT tool, or you can use a sticky notes program like NoteZilla. You should do this because you always run the risk of reading "El cliente tiene que conocer nuestros productos" and then if you decide it sounds better in the plural in English because it's not a specific customer but rather any client who walks in the shop, you run the risk of typing "Customers has to get to know our products". You could slip up and forget that you had decided to use customers in the plural and then translate the Spanish verb in the singular.
Also look for instances of "su" and "sus". As some legal sentences are somewhat long and winding in nature, you might be busy tidying them up and sorting them out and forget if the subject is male, female or an it. Companies are "its", but the people representing them are either a he or a she.
When doing a find and replace, remember that if you are translating from Spanish into English, you cannot assume that you are replacing a mere word or phrase. The word or phrase that was there before the replacement was firmly rooted there much like a plant or tree. If you uproot a term, you must make sure that the context is not disturbed when you replace it with another. An example would be replacing "contract" with agreement. Since you would say "a contract" and "an agreement", if you perform a cursory find and replace, then you will see "a agreement". Someone could quibble about a sentence like that and rightfully so! Likewise, if you change a noun from the singular to the plural or vice versa, you must change the verbs that follow it in all applicable instances. Also, if you are translating from Spanish to English, and the plural is irregular, as in party/parties and you want to replace all instances of "spouse and/or spouses" with "party and/or parties", you have to watch out, because if you type "party" in the replace field, you are going to end up with "partys".
I see proofreading as a sort of insurance. Many times, I spot only a few errors and sometimes none at all. However, the mere fact that I have carefully gone over my work reassures me and lends me extra confidence so I can go on with my day. Now I am going to proofread this post!