I find it's easiest for me to weave a wide web between each of the PCs by using their social connections. After giving the PCs a physical home location (whether a mountain clan, forest home, village, etc), I will try to co-create their background story to include parents, siblings, trainers, apprenticeships, at least a few extended family, close friends, and friends of friends whom they "know of," and who would know their name.
If the PCs themselves don't travel to meet the others on some kind of business trip, you can always bring in a major NPC who is a frequent traveller or trader between the different places.
Ex. The dwarven fighter whose father is a master smith trades crafted weapons to a small but bustling frontier city where he commands higher than usual prices. (the local smiths might not like that he is costing them business!).
An elven ranger provides safety and security for travellers on a particular road through the forest for a small price. One of his regulars might be a travelling merchant.
A human wizard in the town might have need of a rare component and has commissioned a merchant to purchase it from locations far away. After not hearing from the merchant, the wizard might send his apprentice (the PC) to inquire his whereabouts.
Now it isn't too difficult to start with the wizard, give him some funds and perhaps a few scrolls, and then send him in search of the merchant. On his way he discovers the merchant's most regular travel route and talks to some of the other local customers. Discovering that he often carries valuable shipments of dwarven weapons that are highly desireable, but doesn't travel the main roads, it leads the wizard to an encournter with the elven ranger, who upon hearing about his merchant friend decides to accompany the wizard to the dwarven stronghold and discuss further where the merchant travels. Meeting the dwarven smith, he permits his eager son (who has never left the mountain), to accompany the group in search of their merchant friend.
It's impossible to create a good cohesive PC group without digging at least a little into their individual histories and social networks. Think of how you might meet new friends and how people usually meet for the first time and just transfer that into the fantasy campaign.
If you like, try using a social gathering such as a funeral of a prominent figure related to one of the PCs. A person's will can include many things, even a quest. Perhaps the quest is a lifelong journey that the deceased never had a chance to complete; perhaps he left helpful signs along the way.
It can help make things easier if your PCs are all the same race as well. Or all from the same village. Or all know the same swindler/rogue type of PC who has finally duped you all into his idea for getting rich quick!
Greed is almost ALWAYS a good common motivation for PCs. Make them all poor to begin with. In fact make the whole entire city quite poor. Poor people make friends faster in order to survive as a group.
Still need more ideas?