Well, lich is maybe the most item independant hero in the game. With any decent team you usually buy chicken, crow, wards wards wards. Don't get hero kills if another hero can get them (dont let someone escape, thought), you usually play an aggressive babysitter so dont get many creep kills as well, almost only if your (most likely melee) lanemate can not reach it in time. Deny like crazy (start attacking them at half hp, its not that important to get the actual deny, just prevent the lane from pushing), harrass, play agressively, place wards.
Core would be bracer+boots, maybe wand, maybe meka. Luxury is pipe, if needed. If you have many melee heroes, consider getting vlads for them. Point booster cannot hurt, just like str or int treads (guess str make more sense).
Dagger is situational but can be very good, if executed properly (need the right team for this; for example earthshaker with blink also; he blinks in, clears creeps, you follow-up blink and clear the heroes
You can carry flasks for other heroes, dont forget warding. If you happen to have enough money, you can finish aghas. If you have enough money for guinsoo, you are probably doing it wrong (or the game lasts 70 minutes+).
Use your ultimate early in a teamfight, dont wait for THE perfect situation. Later on you just die too fast. Abuse its 60 seconds cooldown on lvl 16; dont hesitate to use it (not on a full creepwave, thought, but dont wait for THE 2 heroes without any allys nearby; those situations are too seldom if your enemy plays well)
Constantly armor your team (the armor is a big factor nowadays, with the removal of many cheap armor items), use meka/pipe when needed
Later on, it is okay for you to die much; if you did your job well, your carry should have good farm on your lane and, due to wards, not be very vulnerable to ganks.