Step number one is to skim the lease and get a sense of what you're explicitly allowed to do. Step number two is to run anything not clearly allowed (or even specifically prohibited) past your landlord -- failure to do so can result in big hits to the security deposit at move-out time.
Once you've got a sense of your options and freedom to improve your space, here are a handful of ways to make your apartment, or rental home, a bit more livable and lovable.
One of the keys to a warm, chic space is making sure that you've got ample light. Add lamps of all sorts, and put everything on a dimmer switch so you can control the mood of the common areas. Lamps can be picked up inexpensively at garage sales, don't require any complicated installation, and have an immediate impact on your space.
If space is your challenge, trade down to smaller furniture. It may seem counterintuitive that a smaller sofa or more minimalist chairs would improve an apartment, but much of what makes a cluttered, tight space feel that way is a lack of room in which to move. Bring the scale of your furnishings down to fit the scale of your space, and you can make a huge impact.
If your landlord is open to the idea, consider recovering your grimey kitchen countertops with a new high-quality laminate -- for less than $100 you can get eight feet of something that resembles marble or granite and will greatly improve the visual impact of your kitchen.
If you've got access and permission to work with your house or apartment's greenspace, spend a bit of money and put in some decorative perennials to warm up the exterior appeal of your home. If you'll be around for a full growing season, considering growing a food garden -- it's cheap, it's satisfying, and it makes a nice conversation piece when you're having those backyard grill-outs.
Floor-to-ceiling curtains can create a dramatic sense of height, and dumping those cheap venetian blinds for something classy and made of fabric can make a big day-to-day impact on your home's look.
Don't underestimate the power of glass or lucite to give your apartment a designer touch. While relatively affordable, transparent tables, chairs, and shelves can lighten up and open up spaces that otherwise feel dark and constricted.
Not enough space? Look into getting a pop-out Murphy bed -- the new models are far more attractive and comfortable than their dangerous, creaky ancestors.
Floating shelves, like glass surfaces, can help give an interior space a light, contemporary, effortless feel. Putting a few of the right items on the right shelves can turn your cluttery knick knacks into a decorating masterstroke... just be careful not to overwhelm the eye with too many items on a single shelf.
Bed risers can be used to bring beds up to a height sufficient to allow for storage space underneath -- it's a great way to keep big storage bins of household goods accessible yet invisible.
If you've got a lot of pairs of shoes and they have a habit of popping up everywhere, get a shoetree, a hanging closet shoe rack, or a shoe rack for your entryway. Now, make sure everyone's shoes stay neatly organized in their new parking places. Voila: More space, and a sense of order that's built from the ground up.