you don't say yes just becouse you feel guilty about saying no ?
well the question is a not directly answearable : you simply evade conduct that would make other people consider you as the one they can abuse into agreeing to help with/buy them anything they want ? the question is also VEEEEERY broad it should clarify who are those people as there is different pattern for saying no/previnting dependance on you for different types of people (kids/spouses/friends etc.) and thier expactation from you.
One thing is clear you cant let others abuse you so you should make them understand that you are not for anyones personal amusement and you have your own life and things that are IMPORTANT to you whitch they should respect, so the question is not how to refuse but how to make others understand that your refusal isn't ill willed, is it ? simply clarify in a conversation that you are a person and have feelings too(that concept shouldn't be too hard for them to swallow) and have your own priorities/needs but you will be there for them if you can, if they get that you can just answear: "no i can't i have to bla bla bla/we can we do it some other time like tommorow ?" and they should understand it .... if you don't want to hurt stranger/classmate straight no is sufficient even if they are making puppy eyes don't get blackmailed ^^ thats all i can say to such broad question