So, I just reached level 45 yesterday. Playing the game around 9-12 hours a day, daily since the game came out. I only work part time which is what allowed me to play the game so much. That being said, here is my rough guide on how to level up, without boosting, or cheating, and being an excellent player in the meantime.
Know Your Role!
I NEED AMO!
Find a class you enjoy. If you like running and gunning and don't like playing too objectively, let me recommend the support class. This class is perfect for people like you. You are not limited to just light machine guns either, you can also use shotguns and SMG's, if you prefer. The reason this class is great for run and gun type players, is that there is not so much focus on "team playing". Not to say you SHOULDN'T be a team player, but it will help ease you into the world of realistic warfare. As the support class, you have access to an array of excellent tools that will help you and your team without having to worry about others too much. These are; the Amo pack, C4, Claymore, Mortar Strike and a variety of high capacity machineguns. To earn points with this class, make sure to drop amo packs around clusters of team mates, C4 unsuspecting tanks, and lay claymores around entrances to objectives. The support class is a highly versitile class suitable for all players. Skilled players and noobs alike should be able to use this class effectively.
MEDIC! MEDIC! Are you a real slayer and objective master? The assault class is for you! The workhorse of the Battlefield army, Assault soldiers are both slayers, and medics. Using this class effectively is very easy and you'll be ranking up in no time. The best way to use this class is to stick with team mates, when they get injured, drop medic packs down to heal them, and when they die, get out your shock pads to revive them. Reviving a team mate will refund your team a ticket, so reviving is extremely important in conquest, and while playing as the attackers in Rush. If you don't want to focus on healing people, the assault class also has access to under-barrels. These range from grenade launchers to shotguns. An extremely useful tactic to use as assault is to equip the smoke grenade launcher. This little baby is incredible. Not only can it cover your team mates as you make your way to objectives, but it can also blind enemy tanks, and cover your ass as you make an escape from a hot area. This however, will not net you any experience points, but remember, if you win a game, you get a winners bonus! WINNING!!!
OH GOD!!! TANK!!! TANK!!! Are you constantly getting blown the f* up by tanks, IFV's and the like? Have no fear, the engineer is here. This class is a great ranking up class, and the worst enemy of vehicles. As an engineer the best way to rank up, is to destroy tanks with your RPG's, and repair friendly vehicles with your repair tool. You can also use the EOD bot to prevent m-com stations blowing up without being near them. Just leave your little robot in a dark corner, and after they plant the bomb, send in the little thing to defuse it. Points galore, and you'll live to tell the tale! Another great tactic to use as the engineer class is to lay anti tank mines on highly traversed roads. Remember, even if you die, the mines don't. So sometimes, even after a good 5 or 10 minutes, you will get a random kill, and the enemy team is a vehicle down. Don't underestimate the fear you put into enemy vehicle users while playing as engineer. Also, here's a pro tactic you can use. When an enemy tank is bothering your team, sneak up behind it, and ruin it by messing it up with your repair tool, just remember to crouch or lay prone so the driver can't see you. When the pilot gets paranoid and confused as to why his tank is dying, quickly get in, and make an escape, then when you are safe, repair the tank, and BANG. Your team now has an extra tank, and the enemy team has one less. The tank is easy to use, and easy to get kills with, so make good use of them.
BOOM! HEADSHOT! So, you're a sniper. A wookie eh? You may be thinking, how on earth can I even remotely contribute to the team while playing recon? But that's the thing, It's now SUPER EASY to get loads of points while playing as recon, the key is in the equipment. Stick your motion sensor near an objective, make good use of the MAV (the flying motion sensor) by hovering around large clusters of enemies and spotting them at the same time (absolute point fest on TDM - and your team WILL win every time). Using the MAV on TDM is where its at for the recon class. I can not stress how ridiculous it is. You will be level 45 in no time. In one round of TDM you can honestly probably net around 20 surveillance ribbons, and bare in mind you get a medal for it every 50 ribbons, worth a whopping 10,000 points! Use the MAV and nothing else in TDM for 4-5 rounds, and you'll be up a good few levels in no time. It's also leading your team to victory guaranteed.
This is my rifle. There are many like it, but this one is mine.
Find a weapon you love, and stick to it. You will recieve a service star for every 100 kills you make with the weapon (worth 2000 points!), and eventually get a dog tag for doing so!
Ribbons and Medals
An easy way to get points is to get ribbons and medals. Check your battlelog online to see which ribbons you need, and then go for them. Most of these are related to getting either a certain amount of kills with specific weapons, or capturing or destroying objectives. Medals come usually after you've received a ribbon 30-50 times. Easy ribbons to get are one's like flag attacker, which require you to capture a certain amount of flags in a game. Playing conquest should net you vast amounts of points.
Squad up
Try to stay in a squad at all times. The game is better when you play with people you know. Staying in a squad means you can spawn on squad members, get squad bonuses and occasionally get the "ACE SQUAD" ribbon. There's no reason NOT to be in a squad, unless you hate human beings, in which case, I can not help you.
DO NOT CHEAT
Do not do it, you will regret it. Boosting is not an easy way to level up, It's an easy way get banned. Dice have boosting tracking algorithms which search for suspicious behavior. So sitting in a corner of a map reviving and killing the same person over and over will get picked up somehow. Not to mention, many people HATE boosters, and if you get caught by someone in the game while boosting, you can except him or her to report you. It's not worth it. I once boosted in Battlefield 2 and the server admin banned me for life, so please understand, boosting will only result in a negative outcome. The last thing you want after reaching level 44 is for your stats to be reset.
That being said, some closing thoughts. Getting points is not hard on Battlefield, It's how long it takes you to get there. Try not to be in a rush, just enjoy the game, and remember the tactics I've mentioned. Good modes for getting lots of points is TDM (fast paced, lots of kills, lots of ribbons), and Conquest (lots of vehicles, lots of ribbons to be gained, lots of team playing bonuses to be made).
I hope this little guide helped you, and remember to have fun! Getting achievements should be fun, not a chore!