I did pretty well with a 4-3-3 with Man Utd. Initially I was using a DM rather than an AMC, and heavily relying on the AML/AMR and DL/DR Wing Backs for the attacking play with my ST either as a DLF or a P - with a Control/Balanced formation. I switched things up at the end of the 1st Season as my AMC's were getting all bitchy about not getting to play FT. So I removed the DM and put in an AMC as an AP. I was pretty happy with it. Won the Premier League with it, but failed to win any of the cups.
My recommendations are:-
ST (Poacher): Neymar/Rooney/Van Persie (VP gets very slack mid-season, so I had to rotate him off, but he gets fed up, so challenge him to play better and he becomes an epic ST)
AMC (Adv. Playmaker): Rooney/Kawasi/Giggs (Giggs gets very upset if you don't inc. him in the first team regularly, so try to rotate him unless you want to bin him. He's pretty good, but he's prone to injury)
AML (Winger): Young/Gotze/Nani/Rodrigo (Young won't play any other position, so that's the only place you can realistically put him. He's not bad, but does tend to mess up his passes if you use him over multiple fixures, so try to rotate him out. Rodrigo is a ST that I got lumbered with by my club, but he's pretty decent on the wing with a bit of coaching. Nani prefers AMR, but he can do AML if you need to rotate Young out. Gotze was a recent aquisition on advice from Van Persie, and he's pretty good. Watch how he tackles though, he's been sinbinned twice in ten appearances.)
AMR (Winger): Nani/Valencia/Van Persie/Kawasi (Nani loves the right, but he does tire easily unless you work on his fitness. Valencia is a natural there, and he's great as he really plays his side well, with good crosses to your other winger. He's good for an offensive team, assuming you can get the ball to him. However, again, with multiple fixures in a row, he's prone to mistakes. Kawasi can play this role, but he tends to dither rather than play the ball, forcing your DR to do most of the work. Van Persie is actually pretty good in this role, but you need to work on your tactics with him in that role, or you'll consistently get warnings in-game about him not blending into the squad, and his play shows it.)
MCL/MCR (Central Midfielder): Carrick/Cabeye/Scholes/Bender/Sandro/Fletcher/Giggs (Admittedly, I've played around with the MC roles a lot, and haven't really found a great mix, as each M(C) offers something different, but in my later games, I've focused on just Central Midfielder here. They're good for getting the ball back from the Defense to the Attackers. Each of those choices there have done something different in the role. I was relying heavily on the AP role here when I was using a DM rather than an AMC, and Ball Winning Midfielder in support of that, and it worked for a while.)
DM (Anchorman): Bender (I only include this because I used it so much, and if you don't want an AMC, a DM is a good replacement. Bender was always on good form here, and suffered none of the multi-fixture flaws that other players are prone to. He's good with the transitional stage of play, and works well with your defence.)
DCL/DCR (Central Defender): Ferdinand/Vidic/Jones/Evans (The defensive team are one of your best assets with Man Utd, as they're very strong in their game. If you use the Balanced tactic with Roaming, they also help with transitional play excellently. However, you need to watch them doing this, as they can often be caught out of position if the other team catch the ball and play the long run up the pitch.)
DL (Wing Back): Evera/Agogo (These two are dynamite in this role, and consistently aid in the advancement in play, and are good for the transitional stages as well. They also run back down into defense as well, which makes them almost saints in my eyes. Evera is prone to multi-match mistakes however, so you may want to alternate matches between them.)
DR (Wing Back): Rafael/Smalling (Smalling is out at the start of the season, which is very annoying, but once you get him, he's a legend. Try to use him more than Rafael, as he can't cope with multi-fixtures at all. Smalling is absolutely everywhere, and where you need him to be at all times. He's been on every winning side I've had, and works extremely well with your AMR for transitional/attacking play.)
GK: De Gee (Always follow the training advice of his coaches, because he gets so much better in the later season if you do. He won the Golden Gloves award at the end of the season because he didn't concede a goal for almost 800 minutes overall. I know a lot had to do with the defense, but he was always jimmy-on-the-spot for me.)
Subs: Make sure you have plenty of subs for your style of play. I always try to have spare wingers, 1 spare striker and a spare defender and midfielder. This is up to you, but make sure you have replacements for the older players as they WILL cop injuries like a banana bruises.
Training: Make sure all your training in the pre-season kick-off is focused on your tactics. Do not change your tactics too much, else it affects your stats considerably and it can take a long time to get them working back up to fluid. After that's up, work hard on your attacking set pieces for match prep and ball control/fitness during the season.
Shouts: Most of the time, I didn't have to use shouts as they could cope pretty well. However, "Pass to Feet" was needed quite a lot, as was "Retain Possession".
Final thoughts: I guess my best advice is to trust your players. Don't overdo it with the team-talks. In fact, I hired Paul Winsper just so he could do my talks (Hired him as a fitness coach, but he has 20 Man Management & 20 Motivation, and it doesn't get any better than that). Hire coaches that focus on finishing as well, as your players tend to shoot long and miss a lot. With your individual training, I would suggest ignoring your assistant for most of it, and work on either crossing or long shots. Finishing is good too, but often a waste of time if the key stats for the role are low.