My previous BF3 post covered most of the basics that an aspiring noob should know, but it was hardly comprehensive. This post covers a few more tips, and is based on my own experiences so it's a little more opinionated (as you'll soon see). That said, I'm sure you'll agree for the most part and will find this info very helpful.
5) Laser Sights/Flashlights... not a fan
I see people rocking Laser Sights all the time, and I just can't fathom it. Flashlights are a little more rare now since DICE nerfed them, but they still show up from time to time. However, this brings me to my point - why do people still use these things? Both give away your position to the enemy very easily, and the blinding effect is really poor. I can honestly say that I have never been killed because of the blinding effect preventing me from killing someone (if anything, they already had the drop on me). In fact, I find the blinding effect easy to compensate for - just shoot towards the middle of the light, or take cover. I think it's pretty telling that when my enemies' laser sights and flashlights have given me more kills than deaths that they're a pretty terrible weapon attachment.
That said, they have their useful situations, but I would never turn off my useless attachment every time I spawn on the off-chance I might need it someday. The laser sight in particular gives you better hip-fire accuracy. However, I personally have learned to just take my silencer/RDS P90 and compensate.
4) Sometimes You Just Have to Charge
If your team and the enemy are stuck at a choke point firing back and forth, your attempts at flanking have failed, and your team is bleeding tickets, sometimes you just have to take the initiative and barrel in there (I'd recommend Assault or Support classes for this, or if you're using a shotgun). I'm not going to lie, 9 times out of 10 you'll probably die (although you may drag a couple people down with you). However, sometimes you'll catch the enemy off guard, especially if you're quick out of the draw. If you're lucky, you'll kill 3+ enemies (possibly the entire opposition on that side of the door) and single-handedly pave a path for your team to advance.
Sometimes, no matter what your team does, it looks like you're totally screwed. The urge to rage-quit can be pretty strong, but it's not over til it's over. And by that I mean that the tide can totally shift unexpectedly. I've had games where my team was getting their asses kicked all game, and then suddenly, with 1 Rush objective left, suddenly we rally and hold the last base from the previously unstoppable foe (and with 300 tickets no less). The reasons for this are simply because if you have a bad team that is losing, then there will be rage-quitters, whose positions will hopefully be filled by skilled players who then turn the tide. In a recent game, I was barely pulling off a 1.0 K/D average in a game (I was about 23 to 20), but my team suddenly was boosted and I managed to rally with a 60 to 40 K/D (pretty damn good considering how badly I was doing).
If you lose a match, then a similar thing will happen with rage-quitters. As a result, the game rebalances the match by taking players on the other team and then puts them on yours (depending on your server settings anyway). In another recent game, my team lost pretty badly, but I knew that the game would rebalance after the match since most players were quitting on my team. Lo-and-behold, in the next match we shut out the enemy and I actually managed to top my Nemesis Victim streak.
Basically, if your team is doing poorly, stick with it. Consider hanging back for a few minutes and picking off the enemy more conservatively. They might turn things around unexpectedly.
2) Change Up Your Tactics
When I play, if I have tried to attack a base from the same approach but have died the last few times, then I believe it is essential to approach it from the opposite vector. For example, if I'm playing Rush on Damavand Peak, and I'm trying to capture the 2nd set of objectives, I generally hang towards the right flank and make my way into the destructible building overlooking the right objective. However, if the enemy has holed up here, and are anticipating us, I will immediately change up my approach, hanging towards the left side of the battlefield instead. Unless the enemy is smart and have spread out evenly, a disproportionate number of troops will be on the right flank, expecting an attack which has stopped coming, allowing you an easier chance at taking the left flank.
Of course, if no approach is working, refer to point #3 and keep at it!
1) Play As a Team!
This ties into my previous list's points, but I didn't really state it quite so explicitly: Battlefield 3 is a team-based shooter, and if you're not playing as a member of a team, you're going to suck. Similarly, if you're going to set the MCOM station but don't have any cover, there's a very good chance you're going to die. BF3 players, support your allies and don't dick around uselessly! I shouldn't have to say this sort of thing so often!