good point here is to review Prerequisites section of WTP Downloads page. All needed products are listed there and must be installed into Eclipse SDK before installing WTP.(download links are also provided for those products). Ensure that all the listed products are installed and running before installing WTP.
In most cases this way works fine to achieve WTP working in your Eclipse SDK.
1) Newer installed WTP from inside Eclipse But... As for me, it's not always true. Basically, required plug-ins can be installed since WTP requires them to be installed. But many of WTP pre-requisites are optional, so, if you want some part of WTP to be installed/work you have to install manually all according prerequisites.
2) I always do the following: Install Eclipse SDK; then download and install all the pre-requisites mentioned in WTP Downloads page; then install some additional products (depending on a project i'm working on); then install WTP itself. The point is to make sure that all the installed plug-ins are loaded after each step. In most cases the WTP doesn't work because some of its pre-requisites isn't installed correctly. Actually I do install only those products which provided as an update site. I'm just unzip all the ZIP'ed products into my Eclipse SDK's /eclipse/ folder one by one and do restart eclipse for each inzipped/instaled product. It's much faster than installing it from inside Eclipse.
Regarding the -clean option... It used to clean plug-ins configuration (some times the old preferences/indexes/configs stored, don't allow a new plug-in to start), so it's usefull sometimes, but doesn't help in case the reason is not the wrong/broken/old plug-in configuration. In case of new installation it cannot help, since there are no saved configs yet.