The first thing to take into consideration when packing for your cruise wear is your destination. Obviously a cruise to the Caribbean and an Alaska cruise will have completely different cruise wear requirements.
For warm weather cruises to destinations like the Caribbean, the Bahamas, South America, Hawaii, etc, you will generally need all of your typical summertime essentials, including swimwear, shorts, short-sleeved shirts, sandals, sneakers, etc. For cruises to destinations with cooler and varying weather conditions from day to day like Alaska, Europe, and Canada/New England in the fall, you will need to consider bringing at least one pair of long pants, a jacket, a raincoat, and a warm sweater, as nights out on deck can often get a little chilly. Mixing and matching tops and bottoms can also save you a lot of room in your suite case.
The second thing to consider is the length of your cruise. If your itinerary is 3-5 days long, packing will be a little simpler than a cruise lasting a week or more. Typically, a cruise lasting 5 nights or less has just one formal night, where a 7 night cruise will usually have 2. Shorter cruises also leave a little more room for "over packing," where longer sailings require a bit more planning.
The most important thing to remember when packing is that all you really need to bring is what you need to feel most comfortable. Most major cruise lines today have a very relaxed dress code, and will never force you to wear something you done want to wear. With the exception of formal nights, virtually anything within reason is most always acceptable.