I game very sneaky, and very deceptively. I've never been able to defeat someone in a head-on battle in any game unless I've had tons of practice, so I use what I call "Sneaky-Ninja-Assassin" style. Since I've noticed that not many people use this strategy(which may be a good thing...), I've decided to share my tips. Use these on any games that your having trouble with.
1.(TBS) Don't declare war until your ships are positioned. This doesn't work as well in multiplayer, but it's perfect in a singleplayer game. Make sure your army is somewhere near the enemies best planets/cities/bases and then declare war. You'll be able to do some severe damage on the first turn.
2.(RTS) Drop small squadrons of troops inside the enemies base before they're ready. This can be totally devastating and it doesn't require a dangerous war of attrition. I works best in games like Starcraft where you can drop cloaked units in.
3.(RTS) Distract your opponent with another attacking force. In a game where both you and your opponent have massive forces but haven't attacked each other, make him/her think that your attack force is small. Send about a sixth of your main force to attack an outpost. The opponent will usually respond by sending their large army to save their city. While they send their army to attack the smaller group, send your big army in to desstroy another outpost, or even better, to flank them.
Strategy is just set apart because you can apply these techniques to most games except for strategy.
1. Snipe. Sniping is what it's all about. You don't get put in any danger while you can kill your opponent with one shot. Easy as that.
2. If the game offers a cloaking character, take it. Nobody will be able to see you unless you want them too. You can combine this with tip one sometimes and it makes for a great combination.
3. Use the unbalanced weapons. This is another one that goes well with tip two. If theres an weapon that lets you spray shots out in multiple directions, use that one. Sneak up close to someone while cloaked and watch as all of the multiple shots go into one person.
4. Abuse bugs. Use bugs to your advantage. For example, in one game I play, Infantry, grenade blasts can go through walls. My character, a cloaking Infiltrator can't take anybody on up-front. I'm also no good at aiming grenades. So if I see someone against a wall. I'll throw a grenade so it will detonate just before it hits the wall, causing the blast to go through damaging and possibly killing the person on the other side of the wall!