With things like love spells you might need to be a little more creative in how you sacrifice. If you are petitioning a deity like Hathor your sacrifice could be giving up beef and maybe milk. This could be either temporary or permanent, or you could swear it off until you can find a nice humane farm to get them from. If you are a proponent of gay marriage you could also donate your time or money toward its legalization.
If neither of those are quite relevant you could just be a little more generous in your definition of "love" - volunteer to help out the mentally disabled or the elderly. Both are quite loving ventures because, as truly sad as it is, most of what your volunteering will be is just talking to them because these people are so damn lonely. I also think that an animal charity would fit well here, too. Animals cannot pay you back. Everything I have given for my cat - the effort, the chores, the money, forgiving the things she has messed up or destroyed - all of that was for her just because I love her and I believe she deserves a happy life. That, I feel, truly captures the nature of what love is.
In the case of spells which improve your appearance or make you a better speaker, there are many ways to sacrifice. There are entire foundations which help children with cleft palates or who have speech impediments. A monetary donation to either would be appropriate. You can also choose a particular article of clothing you think looks especially good (a necklace or a favorite shirt) and give that up, proving your faith in the ritual you are performing. Even better if you were to donate it to a charity that clothes the poor. You could also swear to become an activist for some cause you feel this deity might appreciate. In this case it would almost be like sacrificing the very thing you are asking for - become better looking and charismatic and then use your charisma to help fight for a cause.
I do not know for sure if sacrifices make a ritual or spell more powerful. I know that the workings I have sacrificed in have always worked very well and very quickly, so I tend to assume that it does.
I generally make my sacrifices after the spell unless otherwise compelled to. That way if my sacrifice is something physical like money I can put it on the altar to represent the sacrifice to come. I do try to sacrifice it ASAP afterward, but it's not like Divinity will get mad at me if A.R.F. is closed for the next few days. In fact, that just gives me more time to collect money to donate.
Unless a deity has a specific form of sacrifice they demand, don't worry too much about getting the sacrifice "just right." It's the intention and the spirit of the matter which count most. For example, in Islam during the holy month of Ramadan they fast from sunrise to sunset - no food or drink at all while the sun is out, and part of the reason they do this is for the sake of humility and understanding what it is like to live in a situation where food and clean water are not readily available. This just isn't reasonable for some people though - diabetics, the sick, the elderly, soldiers in a war zone, etc. - these people can't go that long without food or water without risking their lives. In such a case you donate enough money to feed a person for a day. The two sacrifices look rather different, food and money, but the heart of the sacrifice remains the same.