If my Quad-Core 1.2GHz phone with 1GB RAM lags while opening a text message, I have every right to rage. But instead of buying a new phone like most users, I decided to get to the bottom of the problem and solve it.
Root your phone if you haven't already. If you're scared of rooting, read this article and the next thing you know, you'll be Googling “how to root Android.”  
Rule of thumb: usage of resources is directly proportional to the usage of battery. They go hand in hand.
Non-Root Tweaking
It might take some getting used to but lowering your screen brightness to the minimum can save a lot of battery charge. Also, keep the GPS off unless you absolutely need it. Keeping WiFi on at all times is unnecessary; so is having tons of widgets you barely use. Widgets are known to suck your phone's battery dry. Try to keep widget usage to a minimum; use shortcuts instead. Live Wallpapers aren't nice on the resources either.
Free up the RAM
Task Killers are in a grey area, in my opinion. A lot of people hate them and a lot of people love them. I love them and for me, Super Task Killer is a joy to use when I need to free up RAM.
Conserve Battery
SetCPU is a great app for you to underclock your phone. It saves me roughly 20-30 percent of my phone charge. I don't need full 4 cores running at 1.2GHz to send a WhatsApp message. Use this app to set your CPU's clock speed. It's completely safe unless you're mining imaginary virtual currency. Get the Battery Doctor app as well. It's free and it does exactly what the name says.
Get Rid of Bloatware
Did your phone ship with YouTube even though you don't browse YouTube that much? Mine did and I hated it. This is where SDMaid came in. She's a grand app. I used the AppControl function to “freeze” the YouTube app. This stopped the app but didn't uninstall it. If by any freak chance I want to watch a video on YouTube, I'll just unfreeze it. But I doubt it, since Chrome for Android already does the job smoothly.
Social Media Apps... No Thanks
If you wanted me to describe the Facebook for Android app, I'd use superlatives that cannot possibly go on print. It's a resource and battery hog. Plus I get the same app (without push notifications) on my phone's web browser. The trade-off just isn't worth it. Instagram's not a battery hog, though.  
Don't Take Selfies
Really... just don't.