Satellites are vital and we tend to ignore them early on and then it’s too late. So start building satellites as soon as the game stars and keep building them: they give you monthly money, which is exactly what you need, they reduce panic levels and if you manage to fill a continent, you get extra bonuses. Easiest continent to start with: South America!
Try to have 70% engineers and 30% scientists for a smooth operations (and don’t forget about the Workshops!)
You DON’T want to lose any country to Panic, so make sure you keep it under control. Building satellites is one way to do it, but make sure that you also choose your missions carefully and even if one gives you great rewards, go for that where the panic reduction is most needed.
Keep your squad members alive as they level up, they get better and then you lose them and you have to start over.
A trick that some gamers use to make sure that they give it their best to look after their soldiers is to name them Mom, Dad and the names of very good friends.
Snipers are real killing machines focus on leveling them up. They can get the Squad Sight ability at one point, allowing to shoot any Alien that ANY of your squad members sees. And they rarely miss!
Always keep an eye on your research projects, and take full advantage of any upgrades you've earned before heading into a new mission.
Keep adding scientists, engineers and soldiers to your base in roughly equal numbers. If you're playing on a tougher difficulty level, you'll need to be more responsive to the situation as it evolves.
Always have a sniper on your team. Put one of these guys or gals in an elevated position and he / she can turn the tide of battle. Plus, his / her handy battle scanner perk gives you a great view of the battlefield.
Make sure you're changing mission regions regularly, as well as providing satellite coverage to areas that are close to chaos.
The deeper you dig, the more expensive each expansion becomes, so create only the rooms you need.
Focus on the nations who give you the most money each month. If it's a choice between super-rich USA and dirt-poor Nigeria, choose the US.
Rooms of the same class will provide a passive bonus if they're adjacent to each other, so keep that in mind when crafting your overall expansion strategy.