Well, can't say my wallet will survive once Battlecast Urgot goes on sale...
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
The Skin Sale Checklist (Post Update for 8/30/2012)
Originally Created by Shingo Monster.
Edited and Modified by Coreporeal.
PLEASE TAKE NOTICE:
This post has been created to inform potential skin buyers of the possibility of recently released skins that may be featured in an upcoming sale. Newly released skins may go on sale as early as three months after public release; however, this is all subject to change. With the exception of Limited-Edition and Legendary-status skins, every skin released before Mar. 5th has received a sale prior to the latest skin sale announcement, or has been placed in the "unofficial" skin sale Blacklist. The next sale may contain at least 1 skin from the list below.
■ Mar. 05: Battlecast Urgot (975 RP).
■ Mar. 20: Bittersweet Lulu (975 RP), Wicked Lulu (975 RP).
■ Mar. 23: Jurassic Kog'Maw (975 RP).
■ Apr. 01: Fisherman Fizz (975 RP).
■ Apr. 02: Mafia Graves (975 RP).
■ Apr. 06: Battle Bunny Riven (975 RP).
■ Apr. 18: Blood Knight Hecarim (975 RP), Reaper Hecarim (975 RP), Warlord Shen (975 RP).
■ May 01: Frostblade Irelia (975 RP), Rune Wars Renekton (975 RP).
■ May 08: Blight Crystal Varus (975 RP).
■ May 23: Lord Darius (975 RP), Blackthorn Morgana (975 RP), Tyrant Swain (975 RP).
■ May 28: Sad Robot Amumu (975 RP), Darkflame Shyvana (975 RP).
■ May 30: Glaive Warrior Pantheon (975 RP).
All skins not listed here have been placed on sale before, are currently on sale now or have just been announced to be going on sale in a couple of days. The only exceptions to these rules are either Limited-Edition or Legendary-status skins. "Blacklisted" skins are still eligible for skin sales.
With this in mind, it is advisable that you don't buy these skins unless you really want them NOW. (If that's the case, don't complain if you bought it and it happens to go on sale shortly afterwards).
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Riot's "Unofficial" Skin Sale Blacklist
Section created by Coreporeal
These are the only skins that have been absent from any sales since their introduction during Season 2. Skins to be considered to be "blacklisted" have been released to the public for more than six months, and as of yet have not been featured on a sale. The reason for this is, as of yet, unknown.
Listed from the oldest to the most recent.
■ Dec. 29th, 2011: Prototype Viktor (975 RP).
■ Jan. 10th, 2012: Dragonblade Talon (975 RP), Guqin Sona (975 RP), Jade Dragon Wukong (975 RP), Dragon Fist Lee Sin (975 RP).
■ Jan. 17th, 2012: Sabertusk Sejuani (975 RP).
■ Feb. 14th, 2012: Abyssal Nautilus (975 RP), Heartseeker Vayne (975 RP).
■ Feb. 29th, 2012: Nightraven Fiora (975 RP), Bladecraft Orianna (975 RP).
■ Mar. 2nd, 2012: Vandal Gragas (975 RP).
Skin(s) no longer Blacklisted due to the latest skin sale announcement(s): None this sale. Check back next announcement.
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
To view the skins you want before purchasing, along with other info, please visit the links below. Now featuring smilies! 
League of Legends Skin Viewer
A helpful site which allows the user to view the in-game models of any skin, courtesy of Michael Nighingale.
Wanna know how that new Teemo skin looks in-game? Click here. 
How to get a Champ's skin (w/screenshots) by EmissaryShadow.
EncasedShadow (now a Riot employee) started this thread listing all champion skins, links to their in-game screenshots, and availability of the skins, as well as how they may be acquired. He currently keeps updating the thread via his alternate account, EmmisaryShadow. 
Champion Rotation Info Thread by MistressHel.
Like a new skin... for a champion you've never played before? Wish you could test said champ before committing to a purchase? You could try it out when that champ gets placed in the free-week rotation.
Well, when will that happen, if it hasn't already? For that question, MistressHel's "Champion Rotation Info Thread" might have just the answer you seek. And if it doesn't, it may give you an idea as to when it might go into the free-week rotation.
Who knows? It couldn't hurt to look, now could it? 
Champion Sales History and FAQ by Eat Orange Be K.
Skins are always nice, but they can't be bought without purchasing the champion first. But what happens when that Champion you want costs a little too much in IP? An option is to wait for its RP sale.
Eat Orange Be K has made a list of past Champion sales, as well as a general FAQ section that aims to answer questions and misconceptions that may arise during Champion sales.
Please remember, though, he's a little "forgetful", so give him a reminder on updates every once in a while. 
Champion Sales Thread by Woogan.
Be K not updating his thread fast enough? Woogan has gone ahead and done that for you on the EUW server. No, he didn't steal his post, he's just making it easier for others to see what champs have gone on sale, and when. Is that too much to ask for? 
Skin Sales Super-Thread by Coreporeal.
Have you ever looked at a skin sale and wondered, "Has my favorite champion skin ever gone on sale?"
Well, my Skin Sale Super-Thread may just answer that question, as it contains an extensive list of past skin sales, so you may formulate an idea as to when it may just go back on sale again. 
All Skin Sales List by TheReap3r.
Want a little European flavor added to your search for your favorite skin's past sales? Moving to the EU servers and don't want to deal with the NA servers in any way, shape, or form? TheReap3r's got you covered in his thread, which follows a similar principle as mine. Only found in EU West forums.
But please, don't leave our server. We'll miss you! 
Institute of War: The Chamber of Antiquity - (Lore Archives & Other Literary Works) by Sylph.
Sylph has put great effort into amassing a gargantuan mega-thread dedicated to any and all literal works pertaining to League of Legends; Champions, lore, judgements, and much, much more.
Please visit his thread and look around: The world of Runeterra awaits you! 
