1) Look at your idea first. Is it stupid, idiotic, or just plain senseless?
2) Ask yourself the uses. How many does it have? 1-3 or "a lot" does not qualify. It has to be applied to wide ranges of ideas. Reactions with other elements do not count.
3)Does it break saves? If it does then it's an automatic rejection.
4) Use the wiki and/or search bar before posting. If you do not (we will know), it's more than likely to be locked or deleted.
5) Next, check if your idea is too realistic. We don't get paid to make another world.
6) Do not take rejected ideas literal. That means specific wording and minor differences.
7) State it clearly and specifically.
8) Last, but not least, check a mod. A mod might have implemented it, and also don't suggest it should be in the original.

How to suggest it!

Part 1 - The Conception
Now this is the critical point when you think of an idea and is also a key point for shutting down bad ones. Don't post ideas that occur with any of the following thoughts, "Dude, this would be really cool.", "Dude, that was a wicked science lab. That would be so cool.", "Dude, I just thought of a cool idea", or "Powder Toy is missing this element of realism... I should suggest it!". Instead the ideas you suggest should occur with the following thoughts, "Dude, this would have so many uses.", "All these creations could be so much smaller if this existed.", and "This would let us make so many new types of [BLANK]s! It also has plenty of uses!".

Part 2 - The Missing Link
Ok, so you've thought of your suggestion and you've gotten to a computer and are going to suggest it. But wait, is there anything else you could do? Of course! 1) Search your idea, and use plenty of synonyms. 2) Go on Powder Toy and try to think of a/find an existing way to make this using existing tools to make it. 3) Go on IRC(irc.freenode.net or webchat.net freenode.net from a browser, #powder) and ask if it is a good idea or if anyone can think of a way of making it. 4) Check for similar ideas in the "Do not suggest" posts. 5) If it has anything to do with non-GOL life or moving creatures(AI, sticmen, etc) DO NOT POST IT.

Part 3 - Posting
Ok, so as far as you've been able to ascertain your idea is useful and new. Time to complete your suggestion, now it's time to post and brace for the storm of feedback! But first, you need to know how to post. First of all, the title. MAKE IT DESCRIPTIVE!!! I cannot stress this enough your title should do justice to your suggestion. Make sure to include either the name of the element(e.g. TSTN - Tungsten) or a descriptive name of what it does(e.g. PBHL - Powered Black Hole). Do not put something like "New element" or "My Suggestions". Now, onto the actual suggestion. First off include the element name, expanded name, and description at the top("e.g. PBHL - Powered Black Hole - Powered Black Hole. PSCN turns on, NSCN turns off. Creates negative pressure."). Then detail it's attributes such as color, melting and boiling points, state, special properties(e.g. "Midtone gray, solid, doesn't change state, Creates negative pressure and turns 0x000000 black when turned on."). Now finally, and this is important. DETAIL USES. The reader NEEDS to know that your element actually has uses, just in case they don't see any. If you don't you may get many more -1s than if you had included this. A good number to hit is 3, but you should get as many as you can. If you can't think of ATLEAST 3, or atleast 1 major use(e.g. LN2 can be used to reduce temperture, that's about it, but it's a big use). For the PBHL example:
-Equalizing pressure in reactor meltdowns, bunkers, etc.
-Signs(like a not gate sign)
-Converting electricity into pressure.

Extracts from threads: http://powdertoy.co.uk/Discussions/Thread/View.html?Thread=3819 http://powdertoy.co.uk/Discussions/Thread/View.html?Thread=8677