Sincerity

עבודה זרה יח.

תנו רבנן כשחלה רבי יוסי בן קיסמא הלך רבי חנינא בן תרדיון לבקרו אמר לו חנינא אחי <אחי> אי אתה יודע שאומה זו מן השמים המליכוה שהחריבה את ביתו ושרפה את היכלו והרגה את חסידיו ואבדה את טוביו ועדיין היא קיימת ואני שמעתי עליך שאתה יושב ועוסק בתורה [ומקהיל קהלות ברבים] וספר מונח לך בחיקך אמר לו מן השמים ירחמו אמר לו אני אומר לך דברים של טעם ואתה אומר לי מן השמים ירחמו תמה אני אם לא ישרפו אותך ואת ספר תורה באש אמר לו רבי מה אני לחיי העולם הבא אמר לו כלום מעשה בא לידך אמר לו מעות של פורים נתחלפו לי במעות של צדקה וחלקתים לעניים אמר לו אם כן מחלקך יהי חלקי ומגורלך יהי גורלי 

Avoda Zara 18a

When Ribbi Yose b. Kisma was sick, Ribbi Chanina b. Teradyon went to visit him.
He said to him, "Chanina, brother, don't you know that this empire (Rome), was sent by heaven to rule over us- they destroyed His home and burnt His temple and murdered his righteous ones and wore away at His wonders and is still around and about? I heard that you're still teaching Torah to the masses with a Sefer Torah at hand!"
Ribbi Chanina responded: "Mercy will come from Above."
"I'm speaking logically to you and that's how you respond?! I'll be surprised if they don't burn you and your Sefer Torah at stake sooner or later!"
"Ribbi, what's my deal when it comes to the world to come?" Ribbi Chanina asked him.
"Well, do you have any [good] deeds?"
"Once," Ribbi Chanina said, "the money I was gonna blow on booze for Purim got mixed up with the money I intended to give to charity. I distributed the entire sum to the poor."
"If that's so, your share will be the same as mine."

A"A always asks: Here we have Ribbi Chanina, one of the tana'im, who was willing to put his own life on the line to teach Torah to the masses. He belonged to one of the greatest generation of scholars we have had throughout history. Is this really what earned him a portion in olam haba? A couple of bucks that went to charity instead of boozing up?
A"A answers that this wasn't his only good deed. However, it was an act that was performed with complete sincerity. He had no ulterior motives when he distributed this money to charity- no one would have known better and he could have at least taken out part of that sum that he knew he was planning on spending for himself. Harmbam describes Ribbi Chanina's act as being, "בתכלית השלמות האפשרית"  - completely and entirely sincere (Perush Hamishnayot, end of Makkot).
Mitzvot aren't all or nothing- even one act can earn a portion of olam haba. However, sincerity and motive are undoubtedly taken into account.