I agree with user3158900, and I only differ slightly in the way I use it:
{{Form::label('sports', 'Sports')}}
{{Form::select('sports',$aSports,null,array('multiple'=>'multiple','name'=>'sports[]'))}}
However, in my experience the 3rd parameter of the select is a string only, so for repopulating data for a multi-select I have had to do something like this:
<select multiple="multiple" name="sports[]" id="sports">
@foreach($aSports as $aKey => $aSport)
@foreach($aItem->sports as $aItemKey => $aItemSport)
<option value="{{$aKey}}" @if($aKey == $aItemKey)selected="selected"@endif>{{$aSport}}</option>
@endforeach
@endforeach
</select>