Advice, please? How do I come up with a good plot?

First, you must choose a genre.
Then, just listen to music or sit somewhere you feel at peace(museum, library, park, etc) and lose yourself in your own head and write down any ideas that come!
Hope this helped:)

Well, it might take days, even weeks to come up with a good plot. Just be very observant, and change things you see, and soon a good story plot will come to you.

Do things that inspire you, and make you think. Combine two genres if you have to.

Take ideas from from other books, tv shows, or movies, and make them your own. Or take ideas about your life. For me, I play the "What if..." game. Thats how I write, I've been quite successful.

well think of your life and something that has happened in the past say falling down a stair case then bring in your "prince" explain how he caught you in you in his arms and you fell in love and were to be wed but someone jumped your groom-to-be and everyone is searching for the thief so you can have the money for the wedding and afterwards jump on a white horse with your true love and ride off in to the sunset :P sorry i'll admit im out'a it i feel kinda looney today but best-a-luck just take something from your past and think, use your mind, and dream

Well first things first a plot is not something that just magically shows up with a birst of inspiration it takes time you can say 'I'm going to write a book and get it published' that there is your goal as it is with every writer but its not easy you have to nurture your plot with birst of inspiration do you think Stephen King wrote IT or Misery or Carrie or Pet Semetary over night. It took days upon weeks to write it and deside to keep going till he had one finished.
One thing i've learned when in doubt about your plot try writting on or just placing it aside for a later date me i multitask WAY too much when it comes to writing but i do bounce around with my inspiration manias right now i'm trying out a new genre with a novel i started last july. Personally i'd just write as the inspiration comes take note of everything around you even something as little as going to the pool can spark some inspiration for at least a sub plot and sub plots are one of the keys to success with a good plot. I bet you even JKRowling when she was writing Harry Potter had a moment of doubt about her plot. don't give up on a plot that just seems horrible a few days later. The most bizzarre and well loved movies were made up of ideas that ordinarily would be dispelled. i have a friend on the JulNoWriMo forums that is writing about fairy tale land. I recommend you get an account there and contact him his name is agent double O zero .
Personally as far as genre i believe you take it as a sort of guide line and you could always combine your two favorites i'm writting something that is a scifi/fantasy/crime drama this July. I hope my response helps you out
good luck i wish you and every one on this spot to have a never ending amount of inspiration and for the dredded evil Doctor Writers block to stay away

imagination leads to the birth of so much of any creation, however dont allow youself to play judge and jury,allow us that privallage, now get creating, creator;-]

This is what I do:
*Think about something you really like
*Think of you're favorite genre
*Make up a character
*Make up more characters
*Make up a environment
*Listen to songs that realate to your characters & environment, and the future feeling of your story. (ex: My Immortal- Sad, Violent/action- Faint, Horror/Murder- Alice of Human Sacrifice) I actually listen to music while I type. Helps me match the mood.
*Start from there. The plot SHOULD come naturally to you.