James' Tips on Gaining Karma:
1) It is better to respond to the first post than to create the fifth one.
This rule recognizes the fact that most ./ers view their posts in the default ordering. Hence, responses to the first post are placed higher on the page than the fifth post or even the second post. If you are trying to be moderated, the higher you are on the page, the better.
+ This rule is especially applicable on volatile topics such as anything that portrays Linux in any kind of negative light.
+ This rule might not applied if you think that the first post will get modded flamebait or troll or offtopic. If an entry is below the threshold for a viewer, all responses to that entry drop down the the bottom of the list.
+ This rule might not apply if you can't think of any responses to the first post that are relevant to what the first poster said. However, the phenomenon of replying to the first post with something that has nothing to do with the same has observed to be at worst just as ineffective as replying at the bottom of the page, and at best can lead to such spectacular results that everyone forgets the first post entirely.
2) Don't try to be funny.
+ You are rarely funny if you are trying to be; let your natural humor emerge by only being funny by accident.
+ You're probably offensive or just sound stupid. Well, I do when I try to be funny.
+ Most of the things modded funny on Slashdot are not actually funny (this is an axiom).
+ Being modded funny does not help your karma.
+ Being modded flamebait or troll for sounding stupid or being offensive (see above) does hurt your karma.
3) Say intelligent things
Take your time. You are smart, but your first idea might not be your best. Make sure to write your comment the same way you would write a paper in school: each sentence should enhance the idea of the previous, each paragraph should stand alone, together the paragraphs should make a wall of reasoning that the greatest man could never climb.
4) Always reply to people saying retarded things
Even if a million other people have already responded to a brain-dead comment, you still have a good chance of getting +5 Insightful for beating the dead horse. The best way to do it is to explain the reasons why the brain-dead comment is brain-dead, even if they are totally obvious. Also, it is good to sound somewhat scornful, especially if the comment is offensive and/or evinces an attitude that shows a disregard for some person or group.
+ A corollary of the above: If you are going for karma and are not concerned with ideals or principles, don't respond to criticisms of an idea you support if they are numerous and highly modded, unless you have very strong arguments or very strong wording (You don't need both to be modded up.).
5) Edit your comments.
Every spelling and grammar error decreases the chances of your getting modded up by five percent. I just made the statistic up.
A note: I am a hypocrite and do not always follow my own advice. However, through what I have observed, these are all the best things I can tell you.