You can't go wrong with the 3-step formula. From my years of experience of playing moege, I know that all meaningful relationships must:
1) Start with an embarrassing accident
2) Have a donkan and decently attractive1 protag
3) Include cherry blossoms somewhere in the background
Strategically speaking, my first move would be to 'accidentally' smack her in the eye, thus sending her to the hospital. She would then get an eye patch while she recovered - this kills two birds with one stone: if on the off-chance you're really ugly, then she wouldn't be able to tell with the eye patch over her only eye1 (this will also give her a chuu2 look, which is +5 badass points). Anyway, since she is now visually-impaired while she recovers, she will obviously need assistance with everyday things such as walking and eating. Naturally, you volunteer to assist her because the incident was your fault in the first place. This allows you to maximize your time together with her (whether she likes it or not) and lets you hold her hand while you help her walk. I know this is an ingenious strategy, but please, hold your applause until the end.
Now that we have the embarrassing accident and attractiveness out of the way, we need to work on our donkan. You need to be clueless - and I mean really oblivious. If she starts to make romantic advances towards you, the worst thing you could do is notice right away. A professional donkan protag would never realize the heroine's feelings that quickly, if at all. Think of her as a friend for as long as possible (preferably wait until your osananajimi stops waking you up in the morning, because that's when you know for sure that you're on the Sachi route. Rule #8 of moege: all other heroines magically disappear when you're actually on a route). This allows her to fall even deeper in love with you, which ensures a long, lasting relationship.
Oh, and you have to make indirect remarks about her disappointing chest size whenever you get the opportunity. This will make her more self-conscious around you, and suggest to her that you are actually noticing her. Even if you're jeering at her flat chest, heroines long for nothing more than to be noticed by senpai.
At this point, you are finally ready for the confession. However, if it is not currently springtime for you, then that is rather unfortunate. You must wait until the cherry blossoms bloom in spring before you confess. Not only will this maximize the chances of your confession succeeding, but it will make it all the more memorable. It is scientifically-proven that confessions that take place under cherry blossom trees results in longer relationships 82.7% of the time (based on a small sample size).