Marking your score
Making marks in your score is essential for remembering the conductor's instructions and your own observations. Do it both at rehearsals and when practicing at home. The more the better! Just remember to use soft pencil unless it's your own copy.

Score markings are very much a matter of personal taste and vary widely from singer to singer. Experiment, and find a system that works best for you. Below are some useful and commonly used techniques, with an example from an actual score below each set. If you know of others that you'd like to share, let me know using the Contact form.
Navigation and finding your place
Mark your own staff - (e.g. the alto line) so that you can quickly & easily find your part, especially when turning the page. There are lots of ways to do this e.g. putting an arrow or asterisk in the left marging, by underlining the whole line or (if it's your own score) using highlighters - see examples of that later on. I use a circled asterisk, as in the bass part below.
Page turns — mark awkward turns in advance. Below I've added a warning that the bass part comes straight in on the next page. You can also add a preview of the first one or two notes and words, as shown here.
Rehearsal marks and bar numbers - circle them or write them in, particularly if the conductor keeps coming back to practice them e.g. K, 16 and 18 below.

Repeats - circle the beginning and the end of the repeat as below. If the repeat goes back to a previous page, you can write "back to page 2" or whatever helps you to remember the repeat and not get lost!
Shared staves have multiple parts on one staff. This can happen when soprano & alto parts share the upper staff, and tenor & bass the lower (known as "closed score") as shown in the carol below, or when there is a "divisi" (e.g. tenors split into 1st and 2nd tenor) and both parts are written on the same staff. Although the upper and lower part notes will generally have their stems pointing up and down respectively, it may be difficult to pick out your notes. So sometimes I shade over the unwanted part as shown below. It does look a bit ugly, makes it hard to see how the other part relates to yours, and creates more work when erasing your markings! But if it means a more accurate performance with fewer mistakes, I think it's worth it.

Breathing
Breath here — some composers/scores indicate where to breathe, usually with a comma printed above the line. Most don't, so you can add your own comma or tick. I find these hard to see in the heat of rehearsals or performance, so tend to use a large tick that spans the whole staff. See examples of all of this in the bottom line below.
Big breath - I mark this as BB before a particularly long or difficult phrase
No breath - conversely, your conductor will often point out places where you mustn't breathe. As well as noting this as 'no breath' or similar, you can draw a line or arrow connecting the notes/words that you must sing without a breath.
Breathe early - you should always breathe in good time, rather than snatching a breath just before your entry. You can write a reminder to yourself in places where that is particularly important.

Dynamics and expression
Reinforce dynamics - circle or underline where it says p, f and so on. You should also change them, and add new ones, if the conductor requests it.
Crescendo / diminuendo - highlight the hairpins (< and >) and add new ones if the conductor requests it e.g. to grow through a long note, as in this example.
Character reminders - underline and if necessary translate, instructions such as lighter, warm, legato, gentle, energise, or solemnly as in this example.

Pitch
Pitch reminders - circle a tricky note or accidental, and/or indicate why it needs extra care
Finding your note - circle notes in other parts or in your previous phrase and/or add arrows, to remind you how to find your starting note.

Pitch changes - little arrows showing that notes go up, down, or stay the same can be useful when learning tricky phrases with small pitch changes.

Rhythm
Time signature - circle it (or write it in if you've not sung for a page or two) particularly when it changes, so you always know how many beats to count. The example below starts in 3/2 time and then moves into an unusual 4/2 time signature (represented by a cut-C symbol which normally represents 2/2 time).
Counting - write in the beats, with "&" or "+" or "and" if you need to divide the beat in two. You can do this for both notes & rests.

Rhythm traps - circle rhythms that may catch you out, write in counts and/or make a note. In the example below, the phrase circled is normally syncopated, but I've added a warning where it is sung differently ("straight" with no syncopation.)
Danger spots — ! or !! where it is particularly easy to go wrong.

Timing
Tempo changes — mark rit., "a tempo", don’t rush, steady, etc. to remind you not to plough on while everyone else is slowing down!

Watch conductor — perhaps the most important of all the markings! You should do this all the time, but it's particularly important where the conductor needs to bring everyone in (or off) together, or to cue each note precisely, such as at the end of the Hallelujah chorus below.

When not to come in - there are places where it's easy to come in early if you're not paying attention. Mark these places, to avoid an unplanned solo!
Long notes/rests - write in counts (or watch the conductor) for your next entry.

Note length - finishing a note at the right point can be just as important as starting it, especially when the note ends with a consonant. You can use a vertical line to mark the cut-off point. You can also use a line/arrow to remind you to sing up until that point.
Consonant placement - likewise you can mark exactly where the consonant or final sound belongs.

Words & pronunciation
Pronunciation — phonetically spell out difficult words or consonants (e.g. "zyne" as an approximation to the German "sein" in the example below). For words in another language you can also substitute letters with the correct sound (eVige instead of eWige).
Vowel reminders — write the preferred vowel sound where needed
Word stress — underline the stressed syllable e.g. the "Freu" in Freude. You can also put brackets around (or even cross out) syllables that should not be emphasised.
Syllable placement - write each syllable under the corresponding note if the score is not clear on how a word is spread across the notes. This may need clarification from the conductor in order that all singers are in alignment.

Highlighting
Highlight your part - only if you own the score! I find yellow works best (though also use orange, pink etc to distinguish solos and other parts if necessary.) Some singers like to highlight just the words, others just the notes. I do both as I easily get lost! You can also highlight other features that you need to spot easily e.g. dynamics, time signature changes, rehearsal marks.

