See, the very spot your altar sits on will absorb energy and begin to charge up. The longer an area is used for energy work the stronger it becomes. It can eventually become strong enough with its own energy that it acts as a doorway. That's why it's important to close down an altar sight if you move or change locations after a long time.
An unattended energy zone can allow unwanted or negative energies to come and go as they please. Elementals, spiritual creatures created by the energy of the individual Elements can also manifest into physical form through an unclosed circle and wreak havoc! Each of the Elements has a personality type ruler, or overseer, and also the creature associated with that individual Element. Those creatures are the elementals and they get their energy from the element that rules them.
Always remember to eat a bite of food after expending energy at your altar. You can think of it as a communion. Close your rites, kick back, and enjoy a snack. Invite the presence of the deities to remain with you. This can be as casual or as formal as you like. The Gods enjoy a good picnic as much as fine dining! Reserve a small bit of your snack as an offering. Then, when you're all cleaned up and your tools are stowed away in your magick box, take the offering outside. Find a special place to set your offerings each time and this too will store energy just like your altar spot. Remember, the Gods come to us in many forms. Don't put out birdseed as an offering and then get upset if the neighbor's cat eats one of the birds! Even the larvae of the common fly performs service to the Gods by consuming an offering. Like attracts like, right? So if you make a sacrifice, one will be made for you in turn.