Many of the high-level choices made in XCOM: Enemy Unknown involve making short term vs long term decisions. This guide is merely a template which outlines a strategy which provides a well-rounded difficulty curve throughout the game.
Do not skip developing Tier 2 technologies (lasers, Carapace Armor, etc). Make sure you have many of these technologies integrated into your squad by the 16th of month 2 (in time for the first terror mission) . Have Laser Cannons on your interceptors by month 3.
While panic levels may necessitate mixed satellite coverage, prioritize earning Asia and Africa continent bonuses (highly suggest Asia as starting location). South America should be next priority. Aside from grabbing US/Russia for quick cash, get Europe and North America last and don't worry if a few of their countries drop from the council.
For Enemy Within, it's best to wait until mid-game before implementing Meld technologies. Gene Mods are best used on soldiers you are confident can survive, which is difficult to rely on in the early game. MEC Troopers have poor Aim stat progression and rank up at 50% normal speed, so its best to augment soldiers who are already at a high rank. However, you can feel free to hand out medals as you see fit.
Scientists generally help you gain immediate access to improved technologies which can dramatically help your squad in the early game. Engineers provide a production cost savings and allow more satellite uplinks which will help prepare you better for the late game.
The first few months of the game, fielding extra Heavies can bring a lot of extra power to the table with their rocket, high weapon damage, and early-rank bullet swarm ability. Inversely, while very weak their first few ranks, ColonelSnipers are the most desired soldier type to have later on. However, to have any control over your squad makeup often involves hiring & fielding more troops which is in itself a short-term cost.
UFO Power Sources, UFO Flight Computers, Alien Corpses, and Elerium are all resources which generally don't begin to become restrictive until later in the game. Selling them the first two months will provide you with an all-but-necessary financial boost; however continuing to sell them into month 3-4 could halt your progression in the following months.