This is a really important/insightful post from the After Narcissistic Abuse FB group. I bolded some parts for emphasis (for anyone who needs to skim)
Narcissistic supply — it’s number one on the list of a narcissist’s “must-haves”.
Simply put, narcissistic supply is anything (or anyone) that feeds the narcissist’s ego and keeps them artificially pumped up with the attention, admiration and deference of others.
Narcissists have been described as people who are like balloons that are not completely filled. In order to fill in the void, they must draw from the energy of other people, because they cannot produce the energy themselves.
Narcissists require the energy of others to inflate themselves.
Narcissists need attention (and sometimes pity) and want complete agreement with their wishes. They ignore or oppose people who refuse to supply them with these things, and they turn their attention instead to those who can offer these three things in unlimited supply. This is what’s at the heart of narcissistic supply.
If you come into contact with a narcissist and assume they are well-adjusted, you may compliment them at times and pay them a normal amount of friendly attention. They will then identify you as a potential source of narcissistic supply, and proceed with the game of seeing just how much more they can get from you.
Favored sources of supply…
Some people, such as people with low self-esteem and low assertiveness, provide much more supply than others, and make excellent sources, sometimes finding they have unwittingly attracted more than one narcissist into their life because they are too kind, too soft, and too deferential.
Narcissists, always on the prowl, see the free lunch before them and stick around for more — unless and until they find someone even weaker to feed on. Young children make excellent sources of supply, and some narcissists find it fun to get supply from them. (Others hate children for having needs too much to get their supply from kids.)
The narcissist’s constant fishing for attention/compliments, perhaps for pity, and for deference to their wishes will continue and keep increasing as long as the person they are feeding on does not oppose too strongly or too resolutely.
If you refuse…
Typically, at some point sooner or later, the person being used as a source of narcissistic supply will recognize that the narcissist is asking a lot more than they are actually offering in return, and will start to resist. When they start to resist, the narcissist will become displeased, and attempt to manipulate, tantrum or wheedle their way back into feeding position.
Depending on the narcissist, the resistance can take on any number of forms. However, the narcissist will attempt to do whatever they think will work best on the person they are targeting, and will be generally unrelenting in their efforts to get the supply back or at least to emotionally punish the person who is withholding it.
At times, narcissists will grow tired of someone they have been using as a source of narcissistic supply and will toss them away like an old shoe as they opt for someone newer and less wise to their ways. Many a partner of a narcissist has watched in helpless dismay as the narcissist they have had a relationship with rides off into the sunset with a brand new fool. (This is called “discarding“.)
Avoid being targeted — don’t offer supply!
To avoid becoming a target of a narcissist’s attempts to use you for supply, limit your contact with them, set firm boundaries, ask for reciprocity, don’t flatter them, don’t say yes too often, and remember — an ounce of prevention is worth a pound of cure. Turn down invitations from people who seem to need your praise, attention, pity or obedience.