From HulkSkulker's Knowledge Base, these ones are still valid for 3rd Edition.
"Pincer" Attacks - Although you don't want your genestealers skulking around alone, you should spread them out in a way that could make a marine susceptible to multiple attacks from different sides. Yes, genestealers have a pretty low chance of dying in close combat, but it would be nicer if that chance was 0! The best feeling of all is when the marine would have killed the genestealer, but didn't because the dice roll was only counted as defense!
You need to use a two sided attack on marines. In this way, if you attack a marine from any side other than his front (besides those with Thunder Hammers and Lightning Claws) he is only rolling for defense. Higher marine scores will not kill your genestealer. The trick is to lure him into facing one way either by attacking him, or making him use overwatch in a single direction. If you are able to attack him from any other direction, you just got a free attack.
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Stay right there! - Genestealers have the benefit of a lot of movement. With 6 APs, thats 150% of a marine's movement. Heck, it's the only thing genestealer's have got until they get up close! But don't let that fool you into thinking you should use all your APs every chance you get!
When a marine is in overwatch, he is only allowed to fire if he sees an enemy move. Use discretion when moving! You can get greedy with each step, and risk that one, single genestealer getting killed while all the others sit behind a corner. Or you can move one genestealer up a few spaces, and if he survives, move all the others up behind him using the fact that he blocks LOS. This especially works in rooms, where a single genestealer can move to the center, and if he survives, all remaining genestealers can scuttle in and fill the room! Just watch out for flame throwers!
http://www.geocities.com/hulkskulker/strategygs.htm
More Tips
By Donogh:
It's been a decade since I've played Space Hulk (1st Edition) but these should be in your playbook...
Sometimes it may be in your interest to try to get the Marine to jam, even exposing a victim Genestealer to more shots than necessary. This would be useful in a case where the Marine is at the end of a long corridor, and is at the extreme range of your 6APs, or when you want to tempt the Marine player to eat up his reserve of CPs before your main attack. Try some cheeky 90degree turns...
Try closing the door after you when you move in close to the Marines. It'll cost him more to open it than you, and it'll cut down his range (of course he can shoot it down, but that'll take APs as well)
[By Tas: This is one of Comrade James' tradmark moves too!]
By Ogrefencer:
Another good tactic for the chittering hordes is the old 'T-junction special' - this being when a marine is on overwatch facing up the stem of the T the stealer player, assuming he survives when he moves into the top of the T stem should then stop moving. Any stealers behind him can then either queue up behind him or move across the junction behind the static stealer (and therefore out of LOS). I also like closing on marines and diving just into a side corridor a couple of squares away - it keeps them honest!
Some minor progress, amongst the brewing and domestic slavery (!)
And secondly, some extra bits for my second DA Squad off ebay- an Assault Cannon, and a nice DA Power Sword and bolter combo (for the 2nd Squad's Sgt). 
