It is increasingly important to learn how to think your way through hands and try to understand how another player is viewing your actions. If a player sees you bet a large portion of the pot with there are draws out there, they are going to assume you are trying to protect your hand from that draw. If a player sees you call quickly on a board that is full of draws, they may assume you are on the way so to speak and it might make very little sense to make a large bet if that draw never materializes.
When you are attempting a bluff it is important to note what your opponent is telling you about their hand. When your opponent continually is checking the action over to you or checking back at you, they are suggesting that they do not want to continue much farther in the hand if you apply pressure. If they suddenly make a much weaker bet, it may suggest they are suddenly much less impressed with the strength of their hand, though a craftier player may be trying to trap you. It is just important to note that if your opponent is giving you the right away to try and take away the pot or not. You typically want to try and run bluffs that have the greatest chance of success at the lowest risk to your chip stack. You do this by finding spots to bluff when your opponent already is more willing to concede the hand to you.
Depending on the setting in which you are playing its important to realize that when you are bluffing you are more likely to give off a tell of nervousness, since for most of us, lying is not something that comes naturally. This is interpreted differently in live and online settings, but against good players someone may discern it. Therefore it’s important also to understand your own actions when you are bluffing verses when you have a very strong hand. If you bet a certain way when you “have it” as opposed to when you do not, players that you have played with for long enough will be able to tell the difference between the two. Therefore many people will advise that no matter what you have, act in a similar fashion to keep the amount of information given to your opponents as low as possible. After all you only want to give them information that fits with the idea that you have a better hand than they do and that it is time for them to concede the hand to you.