Here are some very simple steps I suggest everyone considers for themselves, when making the decision to level using battlegrounds.
I cannot stress it enough, how important these are for you. When you level mostly using battlegrounds, you will be short on both gold and gear from instances. Sure, you can run a few instances on the side, but it will not make up for the difference much. Heirlooms act as blues of their equivalent level and improve the same time your level increases, so they will always be up to date.
There are enough heirlooms available to give any class you choose, a weapon, two trinkets, and two pieces of armor. In order of priority, the items go armor, weapon, and trinkets. The armor is first because each gives a 10% increase to all experience gained from quests and the killing of a target or monster that yields honor. In case you are wondering, they stack.
In patch 3.3, we hear from wow.com that there might be new heirlooms available, for data mining has shown that there is an item or spell that provides a 5% experience increase; A little extra XP bonus.
The weapon is important, for obvious reasons, as they are often the most important and sought after items that relate to your dps. Some classes can choose from different weapons even, as is the case of the rogue, depending if you are going for combat or assassination specs.
In my case, I chose the following for my mage;
Notice the fact that three of the listed items are pvp in nature. They are available, not from the Dalaran heirloom vendor, but from within the Wintergrasp fortress, for pvp badges. The rest of the slots for my gear, I mostly buy from the AH or get a high level to quickly run me through a low level instance.
If there is one item I suggest everyone buys, it is the last, pvp trinket that frees you from movement impairing effects. It is an absolute must have for every pvp player and will save you from a whole bunch of grief, since playing pvp is half struggle for control and it is achieved through stuns, roots, etc.
Every player should level his or her First-Aid skill, as high as their current level allows. It is true, you can buy bandages but for me, it is essential that I can create them myself. There is never a shortage of cloth at the AH, and if you level like most do, with quests on the side of BGs, you will also gather some yourself.
As items, bandages are a lifesaver in any battleground match, particularly when short on healers (and you always will be); head to head matches often leave, even if you survive, starving for health, and though you are still standing, the next opponent only needs one arrow or spell, or stinking breath, to put you down.
There are a variety of professions that can be useful to you, particularly crafting professions that allow you to easily put together the best craftable items, to supplement your heirloom gear. However, I decided to choose herbalism and alchemy.
Healing and mana potions, the first more than the second, are crucial for me. The two professions allow me to both gather and prepare my own potions, as well as gain Lifeblood. It has saved me more than once when out running the horde, their flag in hand.
It is very important to learn and understand the capabilities of your class. Particularly how they play in pvp, since when going head to head with other players, the game changes drastically. Few, if any of the skills you had in pve, will apply on battlegrounds. Some abilities are only useful in pvp situations learning what your class can do is the only way to shine.
Also do not let anyone gouge you into picking some select, so called "best" classes. Personally, I enjoy playing less obvious choices and it bores me to tears when I check the match roster and realize five from ten in the enemy team are rogues or hunters. Particularly on certain levels, like the 10-19 branch is infested with players of those classes, since they are the obvious, strongest contestants.
For my mage, I was told to be a poor choice for battlegrounds and that I will be ganked by rogues and warriors of lower level than I am. However, so far, I find rogues less imbalanced and more just annoying, who are incapable of anything else except stealth, backstab, and try kill you before the stun wears off.
If it doesn't, or if I can reveal the rogue with an aoe, the rogue better pray for quick reinforcements before he becomes a jar on my mantle piece.
Just like knowing your class, knowing the terrain and purpose of the battlegrounds you play is equally important. It is not a rare sight, to see people wonder aimlessly on arathi basin, not fully understanding how the battleground works, or know the best ways to infiltrate the enemy base in warsong gulch.
Finding people to play battlegrounds with can cut a lot of frustrations from the road ahead. Pugs by nature are chaotic and usually only go with the one tactics; which is to say, they don't have any. Basically, just zerg the enemies, while trying to accomplish the battleground objectives. After you take part in a match, with clear strategies and roles, you realize how much of success depends on people who are willing to communicate and evolve strategies.
And even if you cannot find any pvp guilds to join, just grouping with a friend or two takes you a long way.