The title is pretty self explanatory. Basically here are a few things that I've learnt since coming back into the game and from reading Battle Reports it seems that people are making these mistakes all the time.
I don't claim to be any sort of 40K guru but these things might make the difference between success and failure. Whilst they're geared to assaulting army like Space Wolves they apply to most 40K armies too.
1. Location, Location, Location
It's important to deploy your units in suitable positions e.g. long fangs somewhere with good cover and good fire lines. However, I'd say it's even more important to position the individual models within your units.
There's the obvious things like put template weapons (i.e. flamers) at the front but in a similar way make sure your meltaguns are up front to give them the best chance of half-range shots. Independent characters should go front and centre too. This will make sure they're always going to take part in an assault. Hiding them in a unit doesn't work as they're going to Pile In before the rest of the unit anyway.
Positioning units as a meatshield is also an effective way of giving a unit a cover save for example fenrisian wolves protecting thunderwolves.
Finally, correct positioning of the individual models means that when removing casualties you can do so without hampering effectiveness or coherency. Put your important models in the middle so that when you remove casualties you're never likely to have to remove them. Bear in mind that when being assaulted you may want to remove the front line of troops to prevent a charge after being shot at.
2. Stay Indoors
It may sound obvious but it's certainly something I didn't learn straight away. Since Space Wolves are predominantly a mechanised army it's important to use your transports correctly. It's very tempting to move 12" towards your target, get out, rapid fire and see what's still standing. Even though your grey hunters are adept at shooting they're far more effective if they charge.
If possible stay in your transport until someone blows it up. It can be surprisingly difficult for most units to de-mech you, especially if you moved 12" and popped smoke. However, if you've moved into a suitable position on the previous turn then by all means get out and charge your opponent. However, disembarking early just means you're wasting the protection of the vehicle. Obviously the Land Raider is an exception to this since you can move and assault.
Carefully decide whether to move your Rhinos 12" to get as close as possible or just 6" so you can shoot. Rune Priests with Lightning/Jaws/Hurricane should be allowed to shoot from the Rhino if possible.
3. Hold Your Fire!
We've all done it. You get your unit into assault range, try to thin out your opponent with a spot of shooting. Your shooting is a bit more effective than you expected so you actually kill a good number of your enemy. Your cunning opponent then removes the front line of troops and you find yourself no longer in range. In his turn he charges you and it doesn't end well!
Judging distances is vital in 40K. Get used to what 6" looks like. It's normally further than you think when being charged and shorter when you're doing the charging! When you move into assault range figure out in your head roughly how effective your shooting will be. If you're just over an inch or two away then by all means shoot but otherwise think carefully about it.
The other thing to consider is terrain. Even if you're only a couple of inches away, shooting can make this 3-4" and then you're relying heavily on your dice roll.
4. Beat Them to it
No matter what army you're playing against, getting the charge is often decisive. Whilst technically you only gain 1 attack (unless you're a blood claw or Ragnar), by denying them a bonus attack it's doubled. It's even more important when considering things like furious charge. Whilst charging obviously isn't as important for Space Wolves as it is for Blood Angels or Orks it's denying your opponent the charging bonus that is key. Get charged by Blood Angels and you're in trouble with I5 and S5. Charge them and see how fragile they can be.
It's a very difficult skill to ensure you get the charge. Transports make this slightly easier as, assuming you don't get de-meched, you can stop your opponent charging you. Otherwise it's a matter of getting the distances right. Remember to consider things like fleet, jump packs ignoring terrain and open-topped vehicles covering a lot of ground and still assaulting.
5. Lambs to the Slaughter
When you're playing aggressively it can be easy to get carried away and charge everything. Even though I've just said that charging is important, it's equally essential that you don't charge unless you have to. If there's an opponent that you've a strong chance of losing the combat to then don't charge. If he wins and you either get wiped out or break then he's free to do what he wants in his turn. If you let him charge you then he's not going to be stuck in combat on your turn and you can pelt him with fire.
Obviously this is a lot easier when you've got Space Wolves and their ability to counter-attack but it's worth considering nonetheless. It's essential for armies that rely on shooting such as IG or Tau, with the exception of a couple of units/scenarios neither of these armies should charge.
Also if you think you can kill a unit entirely with shooting then think carefully about whether you rapid fire or not. Charging in might mean you lose men but you'll gain a free advance in the form of your charge and eventual consolidation. By shooting you don't put your own models at risk but you'll be stationary.
These 5 tips just scratch the surface of the sort of things you need to consider to be good at 40K. I can't pretend for a second that I've mastered any of them. I make mistakes all the time but if you consider these things when you're playing then hopefully you'll make less.
I hope they've helped out and that they weren't just pointing out the obvious!