You may have seen this sort of header while reading fics:

Title: Excellent HP Smut
Author: Prolific
Characters: Draco/Harry
Rating: NC-17
Warnings (Highlight to read): m/m, Spoiler: *Main characters' death (of the "little" variety), non-con shaving of ferret fur*
Summary: Harry opens a post-war no-kill animal shelter and one day an injured white ferret is being brought in...
Author's notes: Highlight warnings for spoilers.

Making spoilers is easy. The trick is to make letters and background the same color. Here is code used for the above entry:

<b>Warnings (Highlight to read)</b>: m/m, Spoiler: *<span style="COLOR: #ffffff; BACKGROUND-COLOR: #ffffff">Top secret spoiler content.</span> *
Warnings (Highlight to read): m/m, Spoiler: *Top secret spoiler content.*


Or, to make it easier:

*<span style="COLOR: #ffffff; BACKGROUND-COLOR: #ffffff">WRITE YOUR SPOILERS HERE</span>*

I used #ffffff color (white) for the spoiler, but it doesn't have to be. You may choose any color as long as it looks good in your journal.
Various color codes can be found here and here. 


Some random examples:

background-color: #66FFFF ; color: #66FFFF
Warnings (Highlight to Read): *Spoil me baby*

background-color: #FFFF99; color: #FFFF99
Warnings (Highlight to Read): *Spoil me more*

background-color: #FFB6C1; color: #FFB6C1
Warnings (Highlight to Read): *Keep spoiling. Mmmmm... :D*


Attention: 
Make sure that: 
1) Your spoiler's font and background colors are the same.
2) The spoiler color is DIFFERENT from the main background of the post! ( Check it out in the main feed view and in the individual post view - press "link" to see)
Also remember that many people view their friends list in their own journal style, so the background may be different from yours. The background they see may be white, for example.