It truly depends on what you want to use it for. I keep candles, a metal tray for burning herbs, an athame, sea salt, a pentacle, an incense burner,and a chalice on my non mobile one :)
My altar changes through the seasons to reflect them. Since Yule is coming, I will have pine branches along with gold and silver candles, pine cones, then my usual chalice, 5" rock I painted and charged long ago with earth symbolism from around the world, my mortar and pestle, my bell and various gemstones placed in locations (four specifically that represent the elements/directions). For Beltaine, fresh flowers. For spring, pussy willow branches. I celebrate spring the first warm day of the year, when the snow has melted and grass is green, not the Spring Equinox. The only important thing about an altar (in my opinion) is that it has representations of the directions/elements, and that it's maintained as "sacred".
You can have also so called theme altars .Altar dedicated to your spirit guides ,angels ,ancestors .I have permanent angel altar and i sometimes set up second when i do my magical work .First one i use for prayers and chants and second one for spells or ritual work .I have always my four primary tools ,plus incense and oil burner ,crystals and offerings sometimes .The way it will look will depend of your intend and tradition .Also you can not go wrong with it as long as you keep theelements balanced on it and you follow your gut feeling how to set it up .If you want to see my altars you can contact me so you can see an example .However remember that altar is a personal thing ,something which reflects you and your state of mind, your dreams and goals to strive for .
Artemesia, you bring up a good point. There's different types of altars. A friend of mine has an altar set up to the archangel Michael in his home which he decorates uses to honor him. He has a completly different altar which he uses for magical workings. The first a is a nice small oak table. The second is a standard black double cabinet which they used in the Golden Dawn. Is your altar meant to honor a specific entity or the seasons etc. Or do you actually want to use it during workings. For the former, I would have a picture or statuette of the entity/deity along with some candles and an altar cloth in the color ascociated with him/her, an incense holder, and any other objects ascociated with whoever you wish to honor. For the latter, like I said, first prepare it by annointing the corners with uncrossing oil, then put whatever tools you need on it. An altar cloth is always a nice thing to have. Not only does it protect the altar but it can have certain symbols stitched on it. You could have multiple cloths of different colors for different purposes like a green cloth for money spells, red for love spells etc. One idea I've seen a couple people do including myself is putting a black pillar on the left side and a white pillar on the right side to symbolist Jakin and Boaz, the two pillars supporting Solomons temple. Others use two sphynx figurines. I used to have two jade Pixius I used but switched to the black and white pillars which I made from the brown things in paper towel rolls.
An artal is a personal thing how you ll built it and for what perpuse as Artemisia said some people have many altars so they can dedicate them to a different God or angel. An altar is good to have candles, something to represent every element, a book of shadows, symbols, some herbs and escense... usually that's what is used in an altar... but you can add other things... as I said it's a personal thing
mine is a candle for spirit, two statues of the dieties i pray to, an incense holder, incense, challice [which i always forget the wine so i need to cut the circle to get it lol] a plate for the cake, my athame, and whatever else is needed for the specific sabbat. for example, since Yule's coming up, my cauldron to light a small yule fire, some evergreen branches, and a red and green table cloth.
Like they say it depends. I change mine randomly. Right now it has a large collection of stones A Statue of Quan Yin and Acorns. A dish of Copper. With a Whoite candle
