Assalamua'alaykum wa rahmatullahi wa barakaatuh,
Well, as many have said, do not let yourselves to be alone too much. How? The first step is you can try to get yourselves involve in good, Islamic activities such as joining programmes held by the masjid or the Muslims' organisation at your place. Once you are involved in an active Muslims' organization, sometimes, the only time that you have for yourselves is to go to bed and sleep at night. Even if you are to study, go form a study group. Always try to think on how you will be among people doing good things instead of just being alone in your room, studying etc.
Trust me in this one matter that once you let yourselves to be involve in so many things - things that are done for Islam - then InshaAllaah you will never have the time for yourselves and even think of doing unpermissible things. Be it only a small sin or worse, a big one. Wa iyyathobillahi minthaalik.
Apart from that, it would be best for us to examine and check deeply on the condition of our heart once in a while (or better yet, every single day). It could be that when we are being engaged too much in worldly matters, we did not realize that we are letting ourselves further and further from Our Rabb. Just like our physical body needs food, so does our heart. Feed our heart with its proper "food" such as dhikrullah, reciting the Ayaat of the Quraan, talk to good, practicing Muslims and try to be friends with the poors. Those are some of the ways recommended by Ula'ma on how to cleanse one's heart and make one Ikhlaas in doing an a'amal.
Take this golden opportunity with Ramadhaan's coming to purify our soul and heart. Fighting against one own nafs is the BIGGEST and HARDEST jihaad, but yet it can be done so long that we seek for help from Allaah Azza wa Jalla, constantly striving against it and making dua'a continuosly.
In the end, inshAllaah, you'll gain the Ajer - a bliss, fruitful reward for leaving something (unpermissible) for Allaah's sake.
Wassalamua'alaykum wa rahmatullah.