I just felt like posting this because I'm bored and I see so many players not do these things. There are 6 tips because I couldn't think of ten because I'm tired. Let the trollfest begin..... Feel free to add/elaborate.
You need to know how to move. Learn techniques like wallbouncing, quick strafing, swat turn cancelling, etc. The more ways you can move your character without being predictable, the harder you will be to shoot and the poorer position it will put your enemy in. There are ways to bend the rules of the traditional character movement. Check your corners before sliding into side cover (you should be checking them anyway). This will save you the aggravation of getting sawed off every ten seconds.
This sounds obvious, but I see people all the time that have excellent movement with their character and have me in a bad spot in a shotty battle but can't aim worth a crap and end up getting killed. How many times have you seen the guy that can wallbounce like lightning but couldn't hit the broad side of a barn. You just stand there and watch him look like an idiot until you've seen enough and instagib him. Learn head angles and center mass angles. This is especially useful when sniping and with rifles, but will help exponentially with all weapons. All characters are the same height; learn the headshot tiers when standing, walking, and roadie running and you will pull off %50 more headshots guaranteed.
You will hear so many people complain about actives it will make your ears bleed but it's one of the more important things in Gears. Learn the timing of the perfect (or at least active) reloads of every weapon, they are all on even second counts. Practicing this will allow you to never have to look at the meter when reloading and be able to focus on your movement and aiming and will help enormously in clutch pressure reloading situations.
4. MAP KNOWLEDGE
Obviously know your maps... Know the hotspots, escape routes, and flanking routes at all times in relation to your location and don't be predictable. Every time I play overpass I see the last man retreat into one of the nade spawns or spawn itself. What do you think is going to happen? You will be fighting 5 people in closed quarters with at least one of them lancer/retro/hammer-ing you and you will die within five seconds even if your cleverly planted nades work. Never trap yourself, especially when you are outnumbered. Knowing the maps extensively will help you to avoid that situation.
5. GET A WINGMAN
It's simple: If you want to win more matches, no matter the gametype, you need to use teamwork. It increases your life expectancy and your team's chance of winning. When I am lone wolf in a match I stick around the best/next best player on my team and watch their back even if they don't know it. You have no idea how many clean up kills you can come across by sticking around a hard target and protecting his flank or vice versa how much easier it is to get kills when you have someone doing this.
6. WEAPON USE
This is not as easy as it sounds to most players believe it or not... Utilize your weapons by playing to their strengths and adjust your playing style when you have a pickup weapon to magnify its strength. How many times have you seen someone pick up snipe then fight CQC as if they are going to headshot multiple opponents? Or fire a boomshot ten miles and miss? Or grab frags and waste both of them trying to kill one person just because they had them in hand? Learn the range and maximum kill efficiency of your loadout (especially) and pickup weapons and you WILL get more kills.