The first question for this is: why not say "I Love You"? You have to decide which category you are in, either the "in a relationship" category or the "looking to start a relationship" category. This will better help you find exactly what you are looking for.
First off, "I love you" has been said so many times by so many people, it is very hard to decide what is lies and what is real. This is usually the reason you want to show it- to help an ailing relationship, to start a new flame, or just because. Showing you care can be as hard as you make it, or it can be as simple as you want it. Either way, this article should help a bit. Always the best way to show you care is to spend some time with the other person.
If you are currently in a relationship, chances are that you have already said that you love him/her. And you know that when this is not enough, your actions speak louder than words.
* For a male showering his affections on a woman: basically, you have to realize that although candy and expensive presents are nice, they are not the way to a woman's heart. For example, if she works at a computer or some kind of office setting where she sits all day, her back is probably killing her. The best thing to do for her when she gets home is give her a backrub. This probably will not work for every woman, as a woman working at a business where she is on her feet all day would probably love to just sit and have a nice hot foot bath.
* Also, you have to listen to her, even if she is yelling at you- just let the words slide off you, because thats probably what they are- just words. Refrain from cutting in, as this will only fuel her tirades.
Basically, you have to anticipate what she will need and have it ready, or lend an ear. It really isn't that hard. Plus, doing some of her housework won't kill you, either!
For women doting on a male: Basically, it is the same as the male with the woman- anticipate needs. Men love backrubs, so massaging his neck while he talks about his day will win huge brownie points. And (I laugh at this) when he is on the computer or reading a book (not while he is doing something he can't pause) gently pull back his head and kiss him, then just walk away. This usually gets the most amusing reaction, and it shows that you are thinking about him, which honestly, makes you smile.
Now with starting up a relationship, always be honest- you do not want to lie, because it will ruin things when the other person gets to know you. Do not invite the other person in your house after a date, because this sends the obvious signal. Sex can wait, especially if you care about this person.
If it does come up, be upfront with your feelings, and your partner. If you explain your reasons, it will probably be taken much better that a flat out no. And don't make excuses- tell them that you really care about them and that you think that it can wait until you think that your relationship is ready for such a big leap.
At the point of starting a relationship, a single flower is both a nice gift and not too expensive. Thoughtful gestures, such as picnics in the park or a hike, something that you'll both enjoy (this is important!) is a great way to show you care.
In all, there are many ways to say " I love you " without actually saying it, and many ways to show you care. And always remember- actions speak louder than words!