Unlockable Skins in Lumines:
There are 39 unlockable skins in this game. You have to play through almost all modes and master most of the game before being able to get the last ones. In the US version of the game you get random set of skins through certain level ranges instead of set levels as described here. But the sets are not completly randomized and the later skins are still obtained in the end.

Urbanization - Get to level 4 in Challenge Mode
Round About - Get to level 8 in Challenge Mode
Slipping - Get to level 12 in Challenge Mode
Shake Ya Body - Get to level 16 in Challenge Mode
Square Dance - Get to level 20 in Challenge Mode
Talk 2 You - Get to level 24 in Challenge Mode
Just... - Get to level 28 in Challenge Mode
I hear the music in my Soul - Get to level 32 in Challenge Mode
Dark side beside the river - Get to level 36 in Challenge Mode
Aback - Get to level 40 in Challenge Mode
Working In The Hole - Get to level 44 in Challenge Mode
Sister Walk - Get to level 48 in Challenge Mode
Da-Di-Do - Get to level 52 in Challenge Mode
Strangers - Get to level 56 in Challenge Mode
Holiday In Summer - Get to level 60 in Challenge Mode
Take A Dog Out A Walk - Get to level 65 in Challenge Mode
Big Elpaso - Get to level 70 in Challenge Mode
My Generation - Get to level 75 in Challenge Mode
Meguro - Get to level 80 in Challenge Mode
Spirits - Get to level 85 in Challenge Mode
Get up and go - Get to level 90 in Challenge Mode
Mekong - Beat level 5 in VS. CPU
Please return my CD - Beat level 3 in VS. CPU
The bird singing in the night - Beat level 4 in VS. CPU
Japanese Form - Beat level 1 in VS. CPU
Auto Mobile Industry - Beat level 2 in VS. CPU
Fly Into The Sky - Get to level 95 in Challenge Mode
Lights - Get to level 100 in Challenge Mode
Whoop-De-Do - Beat level 6 in VS. CPU
The Spy Loves Me - Beat level 7 in VS. CPU
Brash - Beat level 8 in VS. CPU
Chinese restaurant - Beat level 9 in VS. CPU
Moon Beam - Beat level 10 in VS. CPU
45 Degrees - Clear all normal puzzles in Puzzle Mode
Rodent - Unlocked by 10 hours of play time
Morning Beats - Unlocked by 20 hours of play time
Water, Flower & Lights - Get 999,999 in Challenge Mode
Tin Toy - Rewarded after clearing VS. CPU
Prime Factor - Clear all x2 puzzles in Puzzle Mode
Shinin' - Complete Challenge Level 1

Unlockable Characters in Lumines:
Character 47 - Beat level 9 in VS. CPU
Character 48 - Beat level 10 in VS. CPU
Character 46 - Beat level 8 in VS. CPU
Character 45 - Beat level 7 in VS. CPU
Character 44 - Beat level 6 in VS. CPU
Character 42 - Beat level 4 in VS. CPU
Character 43 - Beat level 5 in VS. CPU
Character 41 - Beat level 3 in VS. CPU
Character 40 - Beat level 2 in VS. CPU
Character 38 - Get 500 deletes in Time Attack 600
Character 39 - Beat level 1 in VS. CPU
Character 24 - Get 150 deletes in Time Attack 180
Character 25 - Get 250 deletes in Time Attack 300
Character 23 - Get 50 deletes in Time Attack 60

Easy method for most x2 Puzzles:
The second half of Puzzle mode involves clearing the same figure twice. The figure is considered "cleared" as soon as the timeline passes over the END of it. However, only blocks the timeline passes AFTER you've completed the figure will actually be deleted. If you wait until the timeline has passed over most of the figure before actually placing the final block, you will accomplish the first clear--but only the rightmost edge of the figure will be deleted. For most x2 Puzzles, this makes it ridiculously simple to complete the figure a second time and clear the stage.