Compliment/Complement
Compliment with an 'i' means to say something nice, while complement with an 'e' means to match well with. "I like the cake" is a compliment. "I brought a pie" would be a complement. You can remember because it is I who gives compliments.

Discreet/Discrete
To be discreet is to be secretive or careful (i.e. thieves are discreet when they steal. To be discrete means to be separated (i.e. two different piles of objects are discrete piles). Always look to see if there's a 't' to separate the two 'e's.

Who/Whom
If you want to figure out which one is correct, rephrase the sentence as a question and then answer it. For example, to know the correct use in "Mr. Porter called the author who/whom he met in Portland", change it to "who did he meet in Portland?" and then answer the question. If the answer would be "he", use who; if the answer would be "him", use whom. Mr. Porter met 'him' in Portland, so it should be "Mr. Porter called the author whom he met in Portland." Notice that both 'him' and 'whom' end with 'm'.

Less/Fewer and Amount/Number 
Every sign in the grocery store you see that says, "10 items or less" is grammatically incorrect. It should read "10 items or fewer." Use the word 'fewer' with things you can count, and use the word 'less' with items you cannot. For example, there is less sand at this beach, so there are fewer people here. You can't count 'sand', but you can count people. The same rule is true for amount and number. The small amount of soda in your glass means you should have a smaller number of ice cubes. We can't count 'soda', but we can count ice cubes.

Affect/Effect 
In most cases, 'affect' will be a verb, and 'effect' will be a noun. If the word is preceded by an article (a or the), a preposition (into, for, etc.), or an adjective, the correct choice will be 'effect'. Note that 'effect' can also be a verb if it is being used in the sense that something brought something else into existence. "I affected the melon's price" means that I changed it. "I effected the melon's price" means that I created it.

Precede/Proceed
To precede means to come before, while to proceed means to continue. You can remember this because 'e' comes before 'o' in the alphabet.

Bi-/Semi-
The difference between biannual and semiannual is that bi- means every two years, and semi- means twice a year. Semi- has more letters than bi-, so it occurs more frequently.

Councilor/Counselor
A councilor is a member of a council, while a counselor is one who gives advice. In some ways, you can consider a counselor to be one who sells their advice.

Hanged/Hung
Pictures are hung and people are hanged. It is incorrect to say that, "the outlaw was hung at noon." He was hanged at noon.

Elicit/Illicit 
Detectives try to elicit information from witnesses (it's a verb), while things that are naughty or illegal are illicit (it's an adjective). You can remember because things that are illegal are illicit.

Imply/Infer
If you are implying something, you are giving information. If you are inferring something, you are interpreting information. For example, by creating this list of common errors tested on the SAT, I am implying that you should study grammar, and you can infer that your score will increase if you do.