ME1:
Consort (or, more precisely, renegade options when you ask for payment for quests).
Pursue both Ashley and Liara, then choose "may I have you both" so you get Liara - but at least you tried.
Don't forget to help Gianna and save Shiala.
Conclusion: Liara, Consort, attempted threesome.
ME2:
Talk to Kelly and be nice to her (all top options); talk to her at each opportunity and be nice, one day it will lead to dinner.
Recruit Jack first. I don't remember if you can have renegade sex with her before Horizon but seems like you can.
Go Illium ASAP so you get kiss from Gianna and something from Shiala, but avoid Miranda's LM*.
Samara's recruitment, Samara's LM, attempt to hit on Samara (option appears after several dialogues past her LM so it should be done early, may be available only for Paragons).
Do Tali's recruitment and Tali's LM. Tali should be loyal and remain loyal until SM. Start romance with Tali.
Grunt, Zaeed and Garrus better be recruited and loyal to minimize deaths. Thane changes nothing so he might be left until after SM.
O4 mission, sex scene with Tali.
SM: vents, send Tali there and choose Miranda as Fire Team leader. Oops! Make sure Miranda survived, though.
After SM talk to Kelly. She will send a message to your terminal and might be invited for lap dance. Note: she does this even if you had LI and s/he died at SM. Not sure about what happens after you broke up with LI after O4, so sorry, Tali.
LotSB, reinstate relationships with Liara.
Miranda's LM, start romance with her.
*: one romance before O4 and one after, so you have to choose between romancing Tali and Miranda at this point. Also I don't know if Kelly dances for you if you had LI but broke up, but she definitely dances if LI died, so I went for dead LI - and loyal Miranda cannot die at SM so you have to went with Tali.
Conclusion: Jack, Tali, Miranda, Kelly, hit on Samara, kiss from Gianna, something from Shiala.
ME3:
Will see when we get there. Chobot (Allers) looks to be LI, though.
Spoiler: yes, you can romance Ash.