Some common mistakes
Square brackets
Always use square brackets around your transcriptions in order to distinguish them from ordinary text.
Silent letters
There's no such thing as an unpronounced symbol in a phonetic transcription.
[c]
The IPA symbol [c] represents the sound you make with your tongue body hitting your hard palate. English doesn't use this sound. If you're ever tempted to use [c] in transcribing the speech of a normal English speaker, you're almost certainly wrong.
ng, nk
The sound usually spelled ng has the symbol [ŋ]. You should not add a [ɡ] unless there is actually a [ɡ] pronounced -- does it sound more like singer or like finger ? Clusters spelled nk (and often nc) also usually have this sound: [ŋk].
j
Remember: y is [j] and j is [dʒ].
The A's
Be careful not to confuse "typewritten" [a] and "script" [ɑ]. These sounds are made differently. In some languages, switching one for the other can change the meaning of a word.
Capital letters
Don't use capital letters where English spelling conventions do. The waiter Bill and the bill he brings you are pronounced identically and must have the same transcription. In IPA, smaller versions of the capital letters are often used for completely different sounds. Calling someone [ʙɪl] is generally considered rude.
Unstressed vowels
If an unstressed syllable has a neutral vowel, don't transcribe it with a full vowel because it's spelled that way.