I'll usually download a cool song I hear on the radio, listen to it like crazy for a couple of days, and then stick it in my top rated playlist which is always on shuffle where I listen to it periodically. I've recently been getting a lot more music, so I'll list my overall (like the last year or so) and then a more recent one (like last month or so). I also just got last.fm two weeks ago, so it isn't entirely accurate.
Overall:
1 Red Hot Chili Peppers – Can't Stop 280
2 Red Hot Chili Peppers – Get on Top 260
3 Jack Johnson – If I Had Eyes 206
4 Breaking Benjamin – The Diary of Jane (Single Version) 189
5 Jack Johnson – All At Once 145
6 Red Hot Chili Peppers – My Friends 142
7 Jack Johnson – Middle Man 140
8 Jack Johnson – Sleep Through the Static 136
9 Jack Johnson – Bubble Toes 127
10 Breaking Benjamin – Breath 125
11 Red Hot Chili Peppers – Fight Like a Brave 118
12 Nirvana – Smells Like Teen Spirit 115
13 Anberlin – The Feel Good Drag full track 112
14 Red Hot Chili Peppers – Under the Bridge 109
15 Red Hot Chili Peppers – By the Way 106
The more recent list is:
1: Lifehouse - Broken
2: Matchbox Twenty - Unwell
3: Lifehouse - First Time
4: Maroon 5 - She Will Be Loved
5: Jack Johnson - Staple it Together
6: Jason Mraz - I'm Yours
7: Jack Johnson - It's All Understood
8: 3 Doors Down - Here Without You
9: Puddle of Mudd - We Don't Have to Look Back Now
10: Jack Johnson - If I Had Eyes
11: Jack Johnson - All at Once
12: Red Hot Chili Peppers - Wet Sand
13: Lifehouse - Whatever it Takes
14: System of a Down - Lonely Day
15: Jack Johnson - Sleep Through the Static
My favorite bands are: RHCP, Jack Johnson, 3 Doors Down, Three Days Grace, Breaking Benjamin, Maroon 5, OneRepublic, Lifehouse, a bunch of others.
I have around 110 songs in my top-rated playlist, basically the only one I listen to.