During a multiplayer game your first priority is defending your resource site then the other things follows that priority.The resource site must always under your defensive forces it can be a group of units or it can be some defense buildings like cannons,acid spores,turrets.Locating defense force of the resource site must be wisely choosen to eliminate any threat from your enemy and it also will not prevent your resource flux.You can locate these units/buildings near the mineral  group so the target follow can be easily verified.