Transitions

“Transitions are bridges between what has been read and what is about to be read. Transitions help readers move from sentence to sentence; they also alert readers to more global connections of ideas—those between paragraphs or even larger blocks of text. —The Bedford Reader, 5th edition

Words that can be used to show location: 

above behind by near throughout
across below down off to the right
against beneath in back of onto under
along beside in front of on top of  
among between inside outside  
around beyond into over  

Words that can be used to show time:

while first meanwhile  soon then
after second today later next
at third tomorrow afterward as soon as 
before now next week about when suddenly 
during until yesterday finally  

Word that can be used to compare two things:

likewise also while in the same way
like as similarly  

Words that can be used to contrast two things:

but still although on the other hand
however yet otherwise even though

Words that can be used to emphasize a point:

again truly especially for this reason
to repeat in fact to emphasize  

Words that can be used to conclude or summarize:

finally as a result to sum up in conclusion
lastly therefore all in all because

Words that can be used to add information: 

finally as a result to sum up in conclusion
again another for instance for example
as well besides along with other
next finally in addition  

Words that can be used to clarify:

that is for instance in other words

Fore more information, see section 4d ("Make paragraphs coherent; Providing transitions") of The Bedford Handbook.