The following trick only works between the two indicated vendors. You can usually buy items back from Expat at half the price you sold them to Dr. Michel for. First, successfully complete the side quest involving Dr. Michel (from the Med Clinic) and her blackmailer. After the quest is complete, you will get a discount from all the goods she sells. Start selling her all of your expensive equipment. Then, travel to the Wards Markets, and talk to the merchant named Expat. You can buy all of your items back for cheaper than you sold them for. Return to Dr. Michel, and sell her all of your items again. Repeat this process as many times as desired.
Easy experience points:
To gain extra experience from battles, make sure to get out of the Mako for the final hit. You can weaken the stronger enemies from inside the Mako and then get out to finish them off. You will only get 60% of the experience if you kill enemies with the Mako.
Easy Mako repairs:
If your Mako is badly damaged and you do not want to or cannot spend Omni-gel to repair it, save your game, and reload it. Your Mako should now be totally repaired.
Weapon that does not overheat:
Get a HMWA X assault rifle with Frictionless Materials, Combat Optics X, and Snowblind Rounds X as upgrades.
Saving weapon points:
If you are a Soldier, level up weapons so you can use their bonuses and no further.
For example, carnage is the shotgun's bonus. By doing this, you will have extra points for a desired weapon and points to level everything else completely out.
You do not need the damage and accuracy bonus from the points because when you become a Spectre the best of every weapon becomes unlocked in the Normandy or at the C-SEC requisitions office.