Play for the objective and be a team player

• Doing almost anything related to capturing or defending bases is likely to result in considerable XP gains. Flipping Capture Points provides a lot of XP, and you get bonus XP for staying near them, and killing enemies in range of the Capture Point.
• In general, areas around Capture Points tend to be a more target rich environment, giving you more opportunities to kill enemies and support allies.
• Playing in a squad provides XP bonuses. 
• You gain a 25% XP bonus during Meltdown Alerts, and winning one awards you with up to 250 Certs.

Stay away from low-income playstyles

For example, Stalker Infiltrator, long range sniping and air combat. It is possible to score high with these playstyles, but only for an experienced player.

Participate in Double XP Events

Monitor the PlanetSide 2 News Feed or official PlanetSide 2 Twitter Account to learn about incoming Double XP Events. 

During Double XP Events, all XP income is doubled after modifiers, such as Boosts and Membership. In other words, Double XP doubles their effects as well. 

So having Boosts and/or Membership rolling during those events is a great way to make a ton of certs in a small amount of time. 

Speaking of which, Members get a Double XP Event every month.

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Based on this excellent post by /u/KosViik

1. Stay alive. Going for risky plays is not worth it, long term.

2. You can earn a lot of certs just by killing enemies. However, getting to the level where you can do it consistently takes a lot of effort. You need skill, a bit of luck, a good fight and endurance. This combination comes at a stage of the game where you’re not looking for certfarming advice anymore.

3. Playing support Engineer / Combat Medic earns a lot of certs as well – a bit less than an experienced and skilled veteran on a good streak. It is mostly consistent and painfully effortless. You should choose only one of these classes, whichever you enjoy more, and upgrade their support tools and abilities.

4. Don’t “grind” certs. Watching the cert counter tick painfully slow can drive anyone mad. So you should focus on trying to enjoy the game, and become better at it. 

I wanted to improve my Medic, since I neglected it so much. So I played medic. Experimented with engagement ranges and durations with the assault rifle, gauged my durability compared to HA and Infli (my previous “main” areas), noted which revives are and aren’t worth going for, when to abandon reviving and go kill enemies myself, etc. By the time I was happier about how my Medic gameplay was, I was over 1k certs higher than I started, and not once I checked how much I got.

5. You still should monitor how much certs per hour you’re making, as an indication of how well you are doing, regardless of how much you play each day/week. This will help you identify low-income playstyles so you can stay away from them, and focus on things that feel fun and at the same time earn you a lot of certs.

6. Without Boosts, an average player earns about 60-90 certs per hour, if he’s doing okay and doesn’t pick terrible fights. Above average players go all the way to 100-150. Above that are those who know what they are doing, and are doing it. Boosts push these numbers up further.

PlanetSide 2 can be quite a grindy game in terms of “progression”. The fact that weapons are sideagrades to each other, and have all kinds of tradeoffs, really helps to ease the pain. In the end, this is not something you should worry too much about. Just take a mental note of what you want to buy next, and enjoy the game. In the end, that’s what games are for.