Here's the trick with altars. When you first start out the tendency is to put everything "spiritual" you own on the altar. It generally is pretty cluttered with crystals candles, plants, statuary and figurines etc all thrown together. But once you get going you will accumulate enough things that it just cannot all go on the altar. Here are a couple of guidelines.
Balance: Generally speaking all of the objects should look like they go together. Take a look at September Pixie's altar, it has a definite sense of balance among the objects: lots of glass, bright, clear colors, demure objects... here's a flow. If you have an altar cloth with a bright cat-print pattern, a reproduction of an ancient Minoan snake goddess statue, an artsy modern glass chalice from pottery barn, a plastic incense burner with skulls all over it, and big brass Tibetan bell... it's gonna be a big clutter. They don't have to look like a perfectly matched set; an ancient looking statue will be ok next to a fairly natural looking incense burner.
Active: I tend to like it when some part of the altar says ok I am in use right now. This is easily accomplished with candles, incense or fountains and other objects with a sense of movement. Some people have a piece of jewelry that stays on the altar that they wear when they are present in sacred space.
Nothing mundane on the altar! In order or the space to remain special, never put mail or soda cans or anything else on that altar... not even just for a "little while."
Composition: There should be a sense of layout, even if it is a very casual one. Altars tend to have a central focal point (generally a statue) with objects placed around it in a certain way. Some people (mostly pagans) group objects by elemental association or by gender. Astrologically could work as well.
Change: Obviously if you are doing a spell or ritual the altar setup will reflect that but even general use altars would in some way reflect what is going on in your life right now. If you were working on inner strength or confidence a lion on the altar would be appropriate. If you are cultivating stronger relationships objects grouped in twos are good. Natural objects found that relate to the change of seasons in your area help to keep the altar current.